# vmod\_re2

## Varnish Module for access to the Google RE2 regular expression engine

  - Manual section  
    3

### SYNOPSIS

    import re2;
    
    # regex object interface
    new OBJECT = re2.regex(STRING pattern [, <regex options>])
    BOOL <obj>.match(STRING)
    STRING <obj>.backref(INT ref)
    STRING <obj>.namedref(STRING name)
    STRING <obj>.sub(STRING text, STRING rewrite)
    STRING <obj>.suball(STRING text, STRING rewrite)
    STRING <obj>.extract(STRING text, STRING rewrite)
    INT <obj>.cost()
    
    # regex function interface
    BOOL re2.match(STRING pattern, STRING subject [, <regex options>])
    STRING re2.backref(INT ref)
    STRING re2.namedref(STRING name)
    STRING re2.sub(STRING pattern, STRING text, STRING rewrite
                   [, <regex options>])
    STRING re2.suball(STRING pattern, STRING text, STRING rewrite
                      [, <regex options>])
    STRING re2.extract(STRING pattern, STRING text, STRING rewrite
                       [, <regex options>])
    INT re2.cost(STRING pattern [, <regex options>])
    
    # set object interface
    new OBJECT = re2.set([ENUM anchor] [, <regex options>])
    VOID <obj>.add(STRING [, BOOL save] [, BOOL never_capture] [, STRING string]
                   [, BACKEND backend] [, INT integer] [,SUB sub])
    BOOL <obj>.match(STRING)
    INT <obj>.nmatches()
    BOOL <obj>.matched(INT)
    INT <obj>.which([ENUM select])
    STRING <obj>.string([INT n,] [ENUM select])
    BACKEND <obj>.backend([INT n,] [ENUM select])
    INT     <obj>.integer([INT n] [, ENUM select])
    SUB     <obj>.subroutine([INT n] [, ENUM select])
    BOOL    <obj>.check_call([INT n] [, ENUM select])
    STRING <obj>.sub(STRING text, STRING rewrite [, INT n]
                     [, ENUM select])
    STRING <obj>.suball(STRING text, STRING rewrite [, INT n]
                        [, ENUM select])
    STRING <obj>.extract(STRING text, STRING rewrite [, INT n]
                         [, ENUM select])
    BOOL <obj>.saved([ENUM {REGEX, STR, BE, INT, SUB} which] [, INT n]
                     [, ENUM select])
    VOID <obj>.hdr_filter(HTTP [, BOOL])
    
    # utility function
    STRING re2.quotemeta(STRING)
    
    # VMOD version
    STRING re2.version()

### DESCRIPTION

Varnish Module (VMOD) for access to the Google RE2 regular expression
engine.

Varnish VCL uses the PCRE library (Perl Compatible Regular Expressions)
for its native regular expressions, which runs very efficiently for many
common uses of pattern matching in VCL, as attested by years of
successful use of PCRE with Varnish.

But for certain kinds of patterns, the worst-case running time of the
PCRE matcher is exponential in the length of the string to be matched.
The matcher uses backtracking, implemented with recursive calls to the
internal `match()` function. In principle there is no upper bound to the
possible depth of backtracking and recursion, except as imposed by the
`varnishd` runtime parameters `pcre_match_limit` and
`pcre_match_limit_recursion`; matches fail if either of these limits are
met. Stack overflow caused by deep backtracking has occasionally been
the subject of `varnishd` issues.

RE2 differs from PCRE in that it limits the syntax of patterns so that
they always specify a regular language in the formally strict sense.
Most notably, backreferences within a pattern are not permitted, for
example `(foo|bar)\1` to match `foofoo` and `barbar`, but not `foobar`
or `barfoo`. See the link in `SEE ALSO` for the specification of RE2
syntax.

This means that an RE2 matcher runs as a finite automaton, which
guarantees linear running time in the length of the matched string.
There is no backtracking, and hence no risk of deep recursion or stack
overflow.

The relative advantages and disadvantages of RE2 and PCRE is a broad
subject, beyond the scope of this manual. See the references in `SEE
ALSO` for more in-depth discussion.

#### regex object and function interfaces

The VMOD provides regular expression operations by way of the `regex`
object interface and a functional interface. For `regex` objects, the
pattern is compiled at VCL initialization time, and the compiled pattern
is re-used for each invocation of its methods. Compilation failures (due
to errors in the pattern) cause failure at initialization time, and the
VCL fails to load. The `.backref()` and `.namedref()` methods refer back
to the last invocation of the `.match()` method for the same object.

The functional interface provides the same set of operations, but the
pattern is compiled at runtime on each invocation (and then discarded).
Compilation failures are reported as errors in the Varnish log. The
`backref()` and `namedref()` functions refer back to the last invocation
of the `match()` function, for any pattern.

Compiling a pattern at runtime on each invocation is considerably more
costly than re-using a compiled pattern. So for patterns that are fixed
and known at VCL initialization, the object interface should be used.
The functional interface should only be used for patterns whose contents
are not known until runtime.

#### set object interface

`set` objects provide a shorthand for constructing patterns that consist
of an alternation -- a group of patterns combined with `|` for "or". For
example:

    import re2;
    
    sub vcl_init {
          new myset = re2.set();
      myset.add("foo");   # Pattern 1
      myset.add("bar");   # Pattern 2
      myset.add("baz");   # Pattern 3
    }

`myset.match(<string>)` can now be used to match a string against the
pattern `foo|bar|baz`. When a match is successful, the matcher has
determined all of the patterns that matched. These can then be retrieved
with the method `.nmatches()` for the number of matched patterns, and
with `.matched(n)`, which returns `true` if the `nth` pattern matched,
where the patterns are numbered in the order in which they were added:

    if (myset.match("foobar")) {
        std.log("Matched " + myset.nmatches() + " patterns");
        if (myset.matched(1)) {
            # Pattern /foo/ matched
            call do_foo;
        }
        if (myset.matched(2)) {
            # Pattern /bar/ matched
            call do_bar;
        }
        if (myset.matched(3)) {
            # Pattern /baz/ matched
            call do_baz;
        }
    }

An advantage of alternations and sets with RE2, as opposed to an
alternation in PCRE or a series of separate matches in an if-elsif-elsif
sequence, comes from the fact that the matcher is implemented as a state
machine. That means that the matcher progresses through the string to be
matched just once, following patterns in the set that match through the
state machine, or determining that there is no match as soon as there
are no more possible paths in the state machine. So a string can be
matched against a large set of patterns in time that is proportional to
the length of the string to be matched. In contrast, PCRE matches
patterns in an alternation one after another, stopping after the first
matching pattern, or attempting matches against all of them if there is
no match. Thus a match against an alternation in PCRE is not unlike an
if-elsif-elsif sequence of individual matches, and requires the time
needed for each individual match, overall in proportion with the number
of patterns to be matched.

Another advantage of the VMOD's set object is the ability to associate
strings or backends with the patterns added to the set with the `.add()`
method:

    sub vcl_init {
      new prefix = re2.set(anchor=start);
      prefix.add("/foo", string="www.domain1.com");
      prefix.add("/bar", string="www.domain2.com");
      prefix.add("/baz", string="www.domain3.com");
      prefix.add("/quux", string="www.domain4.com");
    
      new appmatcher = re2.set(anchor=start);
      appmatcher.add("/foo", backend=app1);
      appmatcher.add("/bar", backend=app2);
      appmatcher.add("/baz", backend=app3);
      appmatcher.add("/quux", backend=app4);
    }

After a successful match, the string or backend associated with the
matching pattern can be retrieved with the `.string()` and `.backend()`
methods. This makes it possible, for example, to construct a redirect
response or choose the backend with code that is both efficient and
compact, even with a large set of patterns to be matched:

    # Use the prefix object to construct a redirect response from
    # a matching request URL.
    sub vcl_recv {
        if (prefix.match(req.url)) {
            # Pass the string associated with the matching pattern
            # to vcl_synth.
            return(synth(1301, prefix.string()));
        }
    }
    
    sub vcl_synth {
        # The string associated with the matching pattern is in
        # resp.reason.
        if (resp.status == 1301) {
            set resp.http.Location = "http://" + resp.reason + req.url;
            set resp.status = 301;
            set resp.reason = "Moved Permanently";
        }
    }
    
    # Use the appmatcher object to choose a backend based on the
    # request URL prefix.
    sub vcl_recv {
        if (appmatcher.match(req.url)) {
            set req.backend_hint = appmatcher.backend();
        }
    }

#### regex options

Where a pattern is compiled -- in the `regex` and `set` constructors,
and in functions that require compilation -- options may be specified
that can affect the interpretation of the pattern or the operation of
the matcher. There are default values for each option, and it is only
necessary to specify options in VCL that differ from the defaults.
Options specified in a `set` constructor apply to all of the patterns in
the resulting alternation.

  - `utf8`  
    If true, characters in a pattern match Unicode code points, and
    hence may match more than one byte. If false, the pattern and
    strings to be matched are interpreted as Latin-1 (ISO 8859-1), and a
    pattern character matches exactly one byte. Default is **false**.
    Note that this differs from the RE2 default.

  - `posix_syntax`  
    If true, patterns are restricted to POSIX (egrep) syntax. Otherwise,
    the pattern syntax resembles that of PCRE, with some deviations. See
    the link in `SEE ALSO` for the syntax specification. Default is
    **false**. The options `perl_classes`, `word_boundary` and
    `one_line` are only consulted when this option is true.

  - `longest_match`  
    If true, the matcher searches for the longest possible match where
    alternatives are possible. Otherwise, search for the first match.
    For example with the pattern `a(b|bb)` and the string `abb`, `abb`
    matches when `longest_match` is true, and backref 1 is `bb`.
    Otherwise, `ab` matches, and backref 1 is `b`. Default is **false**.

  - `max_mem`  
    An upper bound (in bytes) for the size of the compiled pattern. If
    `max_mem` is too small, the matcher may fall back to less efficient
    algorithms, or the pattern may fail to compile. Default is the RE2
    default (8MB), which should suffice for typical patterns.

  - `literal`  
    If true, the pattern is interpreted as a literal string, and no
    regex metacharacters (such as `*`, `+`, `^` and so forth) have their
    special meaning. Default is **false**.

  - `never_nl`  
    If true, the newline character `\n` in a string is never matched,
    even if it appears in the pattern. Default is **false**.

  - `dot_nl`  
    If true, then the dot character `.` in a pattern matches everything,
    including newline. Otherwise, `.` never matches newline. Default is
    **false**.

  - `never_capture`  
    If true, parentheses in a pattern are interpreted as non-capturing,
    and all invocations of the `backref` and `namedref` methods or
    functions will lead to VCL faillure (see [ERRORS](#errors)),
    including `backref(0)` after a successful match. Default is
    **false**, except for set objects, for which `never_capture` is
    always true (and cannot be changed), since back references are not
    possible with sets.

  - `case_sensitive`  
    If true, matches are case-sensitive. A pattern can override this
    option with the `(?i)` flag, unless `posix_syntax` is true. Default
    is **true**.

The following options are only consulted when `posix_syntax` is true. If
`posix_syntax` is false, then these features are always enabled and
cannot be turned off.

  - `perl_classes`  
    If true, then the perl character classes `\d`, `\s`, `\w`, `\D`,
    `\S` and `\W` are permitted in a pattern. Default is **false**.

  - `word_boundary`  
    If true, the perl assertions `\b` and `\B` (word boundary and not a
    word boundary) are permitted. Default is **false**.

  - `one_line`  
    If true, then `^` and `$` only match at the beginning and end of the
    string to be matched, regardless of newlines. Otherwise, `^` also
    matches just after a newline, and `$` also matches just before a
    newline. Default is
**false**.

#### new xregex = re2.regex(STRING pattern, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    new xregex = re2.regex(
       STRING pattern,
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Create a regex object from `pattern` and the given options (or option
defaults). If the pattern is invalid, then VCL will fail to load and the
VCC compiler will emit an error message.

Example:

    sub vcl_init {
        new domainmatcher = re2.regex("^www\.([^.]+)\.com$");
        new maxagematcher = re2.regex("max-age\s*=\s*(\d+)");
    
        # Group possible subdomains without capturing
        new submatcher = re2.regex("^www\.(domain1|domain2)\.com$",
                               never_capture=true);
    }

#### BOOL xregex.match(STRING)

Returns `true` if and only if the compiled regex matches the given
string; corresponds to VCL's infix operator `~`.

Example:

    if (myregex.match(req.http.Host)) {
       call do_on_match;
    }

#### STRING xregex.backref(INT ref, STRING fallback)

    STRING xregex.backref(
          INT ref,
          STRING fallback="**BACKREF METHOD FAILED**"
    )

Returns the nth captured subexpression from the most recent successful
call of the `.match()` method for this object in the same client or
backend context, or a fallback string in case the capture fails. Backref
0 indicates the entire matched string. Thus this function behaves like
the `\n` notation in the native VCL functions `regsub` and `regsuball`,
and the `$1`, `$2` ... variables in Perl.

Since Varnish client and backend operations run in different threads,
`.backref()` can only refer back to a `.match()` call in the same
thread. Thus a `.backref()` call in any of the `vcl_backend_*`
subroutines -- the backend context -- refers back to a previous
`.match()` in any of those same subroutines; and a call in any of the
other VCL subroutines -- the client context -- refers back to a
`.match()` in the same client context.

`.backref()` may return `fallback` after a successful match, if no
captured group in the matching string corresponds to the backref number.
For example, when the pattern `(a|(b))c` matches the string `ac`, there
is no backref 2, since nothing matches `b` in the string. The default
value of `fallback` is `"**BACKREF METHOD FAILED**"`, but you may set
another value (such as the empty string).

After unsuccessful matches, `.backref()` invokes VCL failure (see
[ERRORS](#errors)). `.backref()` always fails after a failed match, even
if `.match()` had been called successfully before the failure.

The VCL infix operators `~` and `!~` do not affect this method, nor do
the functions `regsub` or `regsuball`. Nor is it affected by the matches
performed by any other method or function in this VMOD (such as the
`sub()`, `suball()` or `extract()` methods or functions, or the `set`
object's `.match()` method).

`.backref()` invokes VCL failure under the following conditions, even if
a previous match was successful and a substring could have been captured
(see [ERRORS](#errors)):

  - The `fallback` string is undefined, for example if set from an unset
    header variable.
  - The `never_capture` option was set to `true` for this object. In
    this case, even `.backref(0)` fails after a successful match
    (otherwise, backref 0 always returns the full matched string).
  - `ref` (the backref number) is out of range, i.e. it is larger than
    the highest number for a capturing group in the pattern.
  - `.match()` was never called for this object prior to calling
    `.backref()`.
  - There is insufficient workspace for the string to be returned.

Example:

    if (domainmatcher.match(req.http.Host)) {
       set req.http.X-Domain = domainmatcher.backref(1);
    }

#### STRING xregex.namedref(STRING name, STRING fallback)

    STRING xregex.namedref(
          STRING name,
          STRING fallback="**NAMEDREF METHOD FAILED**"
    )

Returns the captured subexpression designated by `name` from the most
recent successful call to `.match()` in the current context (client or
backend).

Named capturing groups are written in RE2 as: `(?P<name>re)`. (Note that
this syntax with `P`, inspired by Python, differs from the notation for
named capturing groups in PCRE.) Thus when `(?P<foo>.+)bar$` matches
`bazbar`, then `.namedref("foo")` returns `baz`.

Note that a named capturing group can also be referenced as a numbered
group. So in the previous example, `.backref(1)` also returns `baz`.

`fallback` is returned when the named reference did not match. The
default fallback is `"**NAMEDREF METHOD FAILED**"`.

Like `.backref()`, `.namedref()` is not affected by native VCL regex
operations, nor by any other matches performed by methods or functions
of the VMOD, except for a prior `.match()` for the same object.

`.namedref()` invokes VCL failure (see [ERRORS](#errors)) if:

  - The `fallback` string is undefined.
  - `name` is undefined or the empty string.
  - The `never_capture` option was set to `true`.
  - There is no such named group.
  - `.match()` was not called for this object.
  - There is insufficient workspace for the string to be returned.

Example:

    sub vcl_init {
      new domainmatcher = re2.regex("^www\.(?P<domain>[^.]+)\.com$");
    }
    
    sub vcl_recv {
      if (domainmatcher.match(req.http.Host)) {
         set req.http.X-Domain = domainmatcher.namedref("domain");
      }
    }

#### STRING xregex.sub(STRING text, STRING rewrite, STRING fallback)

    STRING xregex.sub(
          STRING text,
          STRING rewrite,
          STRING fallback="**SUB METHOD FAILED**"
    )

If the compiled pattern for this regex object matches `text`, then
return the result of replacing the first match in `text` with `rewrite`.
Within `rewrite`, `\1` through `\9` can be used to insert the the
numbered capturing group from the pattern, and `\0` to insert the entire
matching text. This method corresponds to the VCL native function
`regsub()`.

`fallback` is returned if the pattern does not match `text`. The default
fallback is `"**SUB METHOD FAILED**"`.

`.sub()` invokes VCL failure (see [ERRORS](#errors)) if:

  - Any of `text`, `rewrite` or `fallback` are undefined.
  - There is insufficient workspace for the rewritten string.

Example:

    sub vcl_init {
        new bmatcher = re2.regex("b+");
    }
    
    sub vcl_recv {
        # If Host contains "www.yabba.dabba.doo.com", then this will
        # set X-Yada to "www.yada.dabba.doo.com".
        set req.http.X-Yada = bmatcher.sub(req.http.Host, "d");
    }

#### STRING xregex.suball(STRING text, STRING rewrite, STRING fallback)

    STRING xregex.suball(
          STRING text,
          STRING rewrite,
          STRING fallback="**SUBALL METHOD FAILED**"
    )

Like `.sub()`, except that all successive non-overlapping matches in
`text` are replaced with `rewrite`. This method corresponds to VCL
native `regsuball()`.

The default fallback is `"**SUBALL METHOD FAILED**"`. `.suball()` fails
under the same conditions as `.sub()`.

Since only non-overlapping matches are substituted, replacing `"ana"`
within `"banana"` only results in one substitution, not two.

Example:

    sub vcl_init {
        new bmatcher = re2.regex("b+");
    }
    
    sub vcl_recv {
        # If Host contains "www.yabba.dabba.doo.com", then set X-Yada to
        # "www.yada.dada.doo.com".
        set req.http.X-Yada = bmatcher.suball(req.http.Host, "d");
    }

#### STRING xregex.extract(STRING text, STRING rewrite, STRING fallback)

    STRING xregex.extract(
          STRING text,
          STRING rewrite,
          STRING fallback="**EXTRACT METHOD FAILED**"
    )

If the compiled pattern for this regex object matches `text`, then
return `rewrite` with substitutions from the matching portions of
`text`. Non-matching substrings of `text` are ignored.

The default fallback is `"**EXTRACT METHOD FAILED**"`. Like `.sub()` and
`.suball()`, `.extract()` fails if:

  - Any of `text`, `rewrite` or `fallback` are undefined.
  - There is insufficient workspace for the rewritten string.

Example:

    sub vcl_init {
        new email = re2.regex("(.*)@([^.]*)");
    }
    
    sub vcl_deliver {
        # Sets X-UUCP to "kremvax!boris"
        set resp.http.X-UUCP = email.extract("boris@kremvax.ru", "\2!\1");
    }

#### INT xregex.cost()

Return a numeric measurement \> 0 for this regex object from the RE2
library. According to the RE2 documentation:

> ... a very approximate measure of a regexp's "cost". Larger numbers
> are more expensive than smaller numbers.

The absolute numeric values are opaque and not relevant, but they are
meaningful relative to one another -- more complex regexen have a higher
cost than less complex regexen. This may be useful during development
and optimization of regular
expressions.

Example:

    std.log("r1 cost=" + r1.cost() + " r_alt cost=" + r_alt.cost());

### regex functional interface

#### BOOL match(STRING pattern, STRING subject, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    BOOL match(
       STRING pattern,
       STRING subject,
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Like the `regex.match()` method, return `true` if `pattern` matches
`subject`, where `pattern` is compiled with the given options (or
default options) on each invocation.

If `pattern` fails to compile, then VCL failure is invoked (see
[ERRORS](#errors)).

Example:

    # Match the bereq Host header against a backend response header
    if (re2.match(pattern=bereq.http.Host, subject=beresp.http.X-Host)) {
       call do_on_match;
    }

#### STRING backref(INT ref, STRING fallback)

    STRING backref(
       INT ref,
       STRING fallback="**BACKREF FUNCTION FAILED**"
    )

Returns the nth captured subexpression from the most recent successful
call of the `match()` function in the current client or backend context,
or a fallback string if the capture fails. The default `fallback` is
`"**BACKREF FUNCTION FAILED**"`.

Similarly to the `regex.backref()` method, `fallback` is returned if
there is no captured group corresponding to the backref number. The
function is not affected by native VCL regex operations, or any other
method or function of the VMOD except for the `match()` function.

The function invokes VCL failure under the same conditions as the
corresponding method (see [ERRORS](#errors)):

  - `fallback` is undefined.
  - `never_capture` was true in the previous invocation of the `match()`
    function.
  - `ref` is out of range.
  - The `match()` function was never called in this context, or if the
    previous `match()` call failed (returned `false`).
  - The pattern failed to compile for the previous `match()` call.
  - There is insufficient workspace for the captured subexpression.

Example:

    # Match against a pattern provided in a beresp header, and capture
    # subexpression 1.
    if (re2.match(pattern=beresp.http.X-Pattern, bereq.http.X-Foo)) {
       set beresp.http.X-Capture = re2.backref(1);
    }

#### STRING namedref(STRING name, STRING fallback)

    STRING namedref(
       STRING name,
       STRING fallback="**NAMEDREF FUNCTION FAILED**"
    )

Returns the captured subexpression designated by `name` from the most
recent successful call to the `match()` function in the current context,
or `fallback` if the corresponding group did not match. The default
fallback is `"**NAMEDREF FUNCTION FAILED**"`.

The function invokes VCL failure under the same conditions as the
corresponding method (see [ERRORS](#errors)):

  - `fallback` is undefined.
  - `name` is undefined or the empty string.
  - The `never_capture` option was set to `true`.
  - There is no such named group.
  - `match()` was not called in this context, or the previous call
    failed.
  - The pattern failed to compile for the previous `match()` call.
  - There is insufficient workspace for the captured expression.

Example:

    if (re2.match(beresp.http.X-Pattern-With-Names, bereq.http.X-Foo)) {
       set beresp.http.X-Capture = re2.namedref("foo");
    }

#### STRING sub(STRING pattern, STRING text, STRING rewrite, STRING fallback, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    STRING sub(
       STRING pattern,
       STRING text,
       STRING rewrite,
       STRING fallback="**SUB FUNCTION FAILED**",
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Compiles `pattern` with the given options, and if it matches `text`,
then return the result of replacing the first match in `text` with
`rewrite`. As with the `regex.sub()` method, `\0` through `\9` may be
used in `rewrite` to substitute captured groups from the pattern.

`fallback` is returned if the pattern does not match `text`. The default
fallback is `"**SUB FUNCTION FAILED**"`.

`sub()` invokes VCL failure (see [ERRORS](#errors)) if:

  - `pattern` cannot be compiled.
  - Any of `text`, `rewrite` or `fallback` are undefined.
  - There is insufficient workspace for the rewritten
    string.

Example:

    # If the beresp header X-Sub-Letters contains "b+", and Host contains
    # "www.yabba.dabba.doo.com", then set X-Yada to
    # "www.yada.dabba.doo.com".
    set beresp.http.X-Yada = re2.sub(beresp.http.X-Sub-Letters,
                                     bereq.http.Host, "d");

#### STRING suball(STRING pattern, STRING text, STRING rewrite, STRING fallback, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    STRING suball(
       STRING pattern,
       STRING text,
       STRING rewrite,
       STRING fallback="**SUBALL FUNCTION FAILED**",
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Like the `sub()` function, except that all successive non-overlapping
matches in `text` are replace with `rewrite`.

The default fallback is `"**SUBALL FUNCTION FAILED**"`. The `suball()`
function fails under the same conditions as
    `sub()`.

Example:

    # If the beresp header X-Sub-Letters contains "b+", and Host contains
    # "www.yabba.dabba.doo.com", then set X-Yada to
    # "www.yada.dada.doo.com".
    set beresp.http.X-Yada = re2.suball(beresp.http.X-Sub-Letters,
                                        bereq.http.Host, "d");

#### STRING extract(STRING pattern, STRING text, STRING rewrite, STRING fallback, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    STRING extract(
       STRING pattern,
       STRING text,
       STRING rewrite,
       STRING fallback="**EXTRACT FUNCTION FAILED**",
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Compiles `pattern` with the given options, and if it matches `text`,
then return `rewrite` with substitutions from the matching portions of
`text`, ignoring the non-matching portions.

The default fallback is `"**EXTRACT FUNCTION FAILED**"`. The `extract()`
function fails under the same conditions as `sub()` and `suball()`.

Example:

    # If beresp header X-Params contains "(foo|bar)=(baz|quux)", and the
    # URL contains "bar=quux", then set X-Query to "bar:quux".
    set beresp.http.X-Query = re2.extract(beresp.http.X-Params, bereq.url,
                                          "\1:\2");

#### INT cost(STRING pattern, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL never\_capture, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    INT cost(
       STRING pattern,
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL never_capture=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Like the `.cost()` method above, return a numeric measurement \> 0 from
the RE2 library for `pattern` with the given options. More complex
regexen have a higher cost than less complex regexen.

Invokes VCL failure if `pattern` cannot be compiled (see
[ERRORS](#errors)).

Example:

    std.log("simple cost=" + re2.cost("simple")
            + " complex cost=" + re2.cost("complex{1,128}"));

#### new xset = re2.set(ENUM anchor, BOOL utf8, BOOL posix\_syntax, BOOL longest\_match, INT max\_mem, BOOL literal, BOOL never\_nl, BOOL dot\_nl, BOOL case\_sensitive, BOOL perl\_classes, BOOL word\_boundary, BOOL one\_line)

    new xset = re2.set(
       ENUM {none, start, both} anchor=none,
       BOOL utf8=0,
       BOOL posix_syntax=0,
       BOOL longest_match=0,
       INT max_mem=8388608,
       BOOL literal=0,
       BOOL never_nl=0,
       BOOL dot_nl=0,
       BOOL case_sensitive=1,
       BOOL perl_classes=0,
       BOOL word_boundary=0,
       BOOL one_line=0
    )

Initialize a set object that represents several patterns combined by
alternation -- `|` for "or".

Optional parameters control the interpretation of the resulting composed
pattern. The `anchor` parameter is an enum that can have the values
`none`, `start` or `both`, where `none` is the default. `start` means
that each pattern is matched as if it begins with `^` for start-of-text,
and `both` means that each pattern is anchored with both `^` at the
beginning and `$` for end-of-text at the end. `none` means that each
pattern is interpreted as a partial match (although individual patterns
within the set may have either of `^` of `$`).

For example, if a set is initialized with `anchor=both`, and the
patterns `foo` and `bar` are added, then matches against the set match a
string against `^foo$|^bar$`, or equivalently `^(foo|bar)$`.

The usual regex options can be set, which then control matching against
the resulting composed pattern. However, the `never_capture` option
cannot be set, and is always implicitly true, since backrefs and
namedrefs are not possible with sets.

Sets are compiled automatically when `vcl_init` finishes (or when the
deprecated `.compile()` method is called). Compilation fails if any of
the added patterns cannot be compiled, or if no patterns were added to
the set. It may also fail if the `max_mem` setting is not large enough
for the composed pattern. In that case, the VCL load will fail with an
error message (then consider a larger value for `max_mem` in the set
constructor).

Example:

    sub vcl_init {
      # Initialize a regex set for partial matches
      # with default options
      new foo = re2.set();
    
      # Initialize a regex set for case insensitive matches
      # with anchors on both ends (^ and $).
      new bar = re2.set(anchor=both, case_sensitive=false);
    
      # Initialize a regex set using POSIX syntax, but allowing
      # Perl character classes, and anchoring at the left (^).
      new baz = re2.set(anchor=start, posix_syntax=true,
                        perl_classes=true);
    }

#### VOID xset.add(STRING, \[STRING string\], \[BACKEND backend\], \[BOOL save\], \[BOOL never\_capture\], \[INT integer\], \[SUB sub\])

    VOID xset.add(
          STRING,
          [STRING string],
          [BACKEND backend],
          [BOOL save],
          [BOOL never_capture],
          [INT integer],
          [SUB sub]
    )

Add the given pattern to the set. If the pattern is invalid, `.add()`
fails, and the VCL will fail to load, with an error message describing
the problem.

If values for the `string`, `backend`, `integer` and/or `sub` parameters
are provided, then these values can be retrieved with the `.string()`,
`.backend()`, `.integer()` and `.subroutine()` methods, respectively, as
described below. This makes it possible to associate data with the added
pattern after it matches successfully. By default the pattern is not
associated with any such value.

If `save` is true, then the given pattern is compiled and saved as a
`regex` object, just as if the `regex` constructor described above is
invoked. This object is stored internally in the `set` object as an
independent matcher, separate from "compound" pattern formed by the set
as an alternation of the patterns added to it. By default, `save` is
**false**.

When the `.match()` method on the set is successful, and one of the
patterns that matched is associated with a saved internal `regex`
object, then that object may be used for subsequent method invocations
such as `.sub()` on the set object, whose meanings are the same as
documented above for `regex` objects. Details are described below.

When an internal `regex` object is saved (i.e. when `save` is true), it
is compiled with the same options that were provided to the set object
in the constructor. The `never_capture` option can also be set to false
for the individual regex, even though it is implicitly set to true for
the full set object (default is false).

`.add()` MUST be called in `vcl_init`, and MAY NOT be called after
`.compile()`. VCL failure is invoked if `.add()` is called in any other
subroutine (see [ERRORS](#errors)). If it is called in `vcl_init` after
`.compile()`, then the VCL load will fail with an error message. Note
that `.compile()` is now unnecessary and deprecated.

When the `.matched(INT)` method is called after a successful match, the
numbering corresponds to the order in which patterns were added. The
same is true of the INT arguments that may be given for methods such as
`.string()`, `.backend()` or `.sub()`, as described below.

Example:

    sub vcl_init {
        # literal=true means that the dots are interpreted as literal
        # dots, not "match any character".
        new hostmatcher = re2.set(anchor=both, case_sensitive=false,
                                  literal=true);
        hostmatcher.add("www.domain1.com");
        hostmatcher.add("www.domain2.com");
        hostmatcher.add("www.domain3.com");
    }
    
    # See the documentation of the .string() and .backend() methods
    # below for uses of the parameters string and backend for .add().

#### VOID xset.compile()

**This method is deprecated**, and will be removed in a future version.
`.compile()` may be omitted, since compilation now happens automatically
when `vcl_init` finishes.

Compile the compound pattern represented by the set -- an alternation of
all patterns added by `.add()`.

Compilation may fail for any of the reasons described for automatic
compilation of set objects as described above.

`.compile()` MUST be called in `vcl_init`, and MAY NOT be called more
than once for a set object. VCL failure is invoked if it is called in
any other subroutine. If it is called a second time in `vcl_init`, the
VCL load will fail.

#### BOOL xset.match(STRING)

Returns `true` if the given string matches the compound pattern
represented by the set, i.e. if it matches any of the patterns that were
added to the set.

The matcher identifies all of the patterns that were added to the set
and match the given string. These can be determined after a successful
match using the `.matched(INT)` and `.nmatches()` methods described
below.

A match may also fail (leading to VCL failure) if the internal memory
limit imposed by the `max_mem` parameter in the constructor is exceeded.
(With the default value of `max_mem`, this ordinarily requires very
large patterns and/or a very large string to be matched.) Since about
version 2017-12-01, the RE2 library reports this condition. If matches
fail due to the out-of-memory condition, increase the `max_mem`
parameter in the constructor.

Example:

    if (hostmatcher.match(req.http.Host)) {
       call do_when_a_host_matched;
    }

#### BOOL xset.matched(INT)

Returns `true` after a successful match if the `nth` pattern that was
added to the set is among the patterns that matched, `false` otherwise.
The numbering of the patterns corresponds to the order in which patterns
were added in `vcl_init`, counting from 1.

The method refers back to the most recent invocation of `.match()` for
the same object in the same client or backend context. It always returns
`false`, for every value of the parameter, if it is called after an
unsuccessful match (`.match()` returned `false`).

`.matched()` invokes VCL failure (see [ERRORS](#errors)) if:

  - The `.match()` method was not called for this object in the same
    client or backend scope.
  - The integer parameter is out of range; that is, if it is less than 1
    or greater than the number of patterns added to the set.

Example:

    if (hostmatcher.match(req.http.Host)) {
        if (hostmatcher.matched(1)) {
            call do_domain1;
        }
        if (hostmatcher.matched(2)) {
            call do_domain2;
        }
        if (hostmatcher.matched(3)) {
            call do_domain3;
        }
    }

#### INT xset.nmatches()

Returns the number of patterns that were matched by the most recent
invocation of `.match()` for the same object in the same client or
backend context. The method always returns 0 after an unsuccessful match
(`.match()` returned `false`).

If `.match()` was not called for this object in the same client or
backend scope, `.nmatches()` invokes VCL failure (see
[ERRORS](#errors)).

Example:

    if (myset.match(req.url)) {
        std.log("URL matched " + myset.nmatches()
                + " patterns from the set");
    }

#### INT xset.which(ENUM {FIRST, LAST, UNIQUE} select=UNIQUE)

Returns a number indicating which pattern in a set matched in the most
recent invocation of `.match()` in the client or backend context. The
number corresponds to the order in which patterns were added to the set
in `vcl_init`, counting from 1.

If exactly one pattern matched in the most recent `.match()` call (so
that `.nmatches()` returns 1), and the `select` ENUM is set to `UNIQUE`,
then the number for that pattern is returned. `select` defaults to
`UNIQUE`, so it can be left out in this case.

If more than one pattern matched in the most recent `.match()` call
(`.nmatches()` \> 1), then the `select` ENUM determines the integer that
is returned. The values `FIRST` and `LAST` specify that, of the patterns
that matched, the first or last one added via the `.add()` method is
chosen, and the number for that pattern is returned.

`.which()` invokes VCL failure (see [ERRORS](#errors)) if:

  - `.match()` was not called for the set in the current client or
    backend transaction, or if the previous call returned `false`.
  - More than one pattern in the set matched in the previous `.match()`
    call, but the `select` parameter is set to `UNIQUE` (or left out,
    since `select` defaults to `UNIQUE`).

Examples:

    sub vcl_init {
        new myset = re2.set();
        myset.add("foo"); # Pattern 1
        myset.add("bar"); # Pattern 2
        myset.add("baz"); # Pattern 3
        myset.compile();
    }
    
    sub vcl_recv {
        if (myset.match("bar")) {
            # myset.which() returns 2.
        }
        if (myset.which("foobaz")) {
            # myset.which() fails and returns 0, with a log
            #               message indicating that 2 patterns
            #               matched.
            # myset.which(FIRST) returns 1.
            # myset.which(LAST) returns 3.
        }
        if (myset.match("quux")) {
            # ...
        }
        else {
            # myset.which() fails and returns 0, with either or
            # no value for the select ENUM, with a log message
            # indicating that the previous .match() call was
            # unsuccessful.
        }

#### STRING xset.string(INT n, ENUM select)

    STRING xset.string(
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns the string associated with the nth pattern added to the set, or
with the pattern in the set that matched in the most recent call to
`.match()` in the same task scope (client or backend context). The
string set with the `string` parameter of the `.add()` method during
`vcl_init` is returned.

The pattern is identified with the parameters `n` and `select` according
to these rules, which also hold for all further `set` methods documented
in the following.

  - If `n` \> 0, then select the nth pattern added to the set with the
    `.add()` method, counting from 1. This identifies the nth pattern in
    any context, regardless of whether `.match()` was called previously,
    or whether a previous call returned `true` or `false`. The `select`
    parameter is ignored in this case.
  - If `n` \<= 0, then select a pattern in the set that matched
    successfully in the most recent call to `.match()` in the same task
    scope. Since `n` is 0 by default, `n` can be left out for this
    purpose.
  - If `n` \<= 0 and exactly one pattern in the set matched in the most
    recent invocation of `.match()` (and hence `.nmatches()` returns 1),
    and `select` is set to `UNIQUE`, then select that pattern. `select`
    defaults to `UNIQUE`, so when exactly one pattern in the set
    matched, both `n` and `select` can be left out.
  - If `n` \<= 0 and more than one pattern matched in the most recent
    `.match()` call (`.nmatches()` \> 1), then the selection of a
    pattern is determined by the `select` parameter. As with `.which()`,
    `FIRST` and `LAST` specify the first or last matching pattern added
    via the `.add()` method.

For the pattern selected by these rules, return the string that was set
with the `string` parameter in the `.add()` method that added the
pattern to the set.

`.string()` invokes VCL failure (see [ERRORS](#errors)) if:

  - The values of `n` and `select` are invalid:
      - `n` is greater than the number of patterns in the set.
      - `n` \<= 0 (or left to the default), but `.match()` was not
        called earlier in the same task scope (client or backend
        context).
      - `n` \<= 0, but the previous `.match()` call returned `false`.
      - `n` \<= 0 and the `select` ENUM is `UNIQUE` (or default), but
        more than one pattern matched in the previous `.match()` call.
        This can be avoided by checking for `.nmatches() == 1`.
  - No string was associated with the pattern selected by `n` and
    `select`; that is, the `string` parameter was not set in the
    `.add()` call that added the pattern. This can be avoided by
    checking the `.saved()` method (see below).

Examples:

    # Match the request URL against a set of patterns, and generate
    # a synthetic redirect response with a Location header derived
    # from the string assoicated with the matching pattern.
    
    # In the first example, exactly one pattern in the set matches.
    
    sub vcl_init {
        # With anchor=both, we specify exact matches.
        new matcher = re2.set(anchor=both);
        matcher.add("/foo/bar", "/baz/quux");
        matcher.add("/baz/bar/foo", "/baz/quux/foo");
        matcher.add("/quux/bar/baz/foo", "/baz/quux/foo/bar");
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            # Confirm that there was exactly one match
            if (matcher.nmatches() != 1) {
                return(fail);
            }
            # Divert to vcl_synth, sending the string associated
            # with the matching pattern in the "reason" field.
            return(synth(1301, matcher.string()));
        }
    }
    
    sub vcl_synth {
        # Construct a redirect response, using the path set in
        # resp.reason.
        if (resp.status == 1301) {
            set resp.http.Location
                = "http://otherdomain.org" + resp.reason;
            set resp.status = 301;
            set resp.reason = "Moved Permanently";
            return(deliver);
        }
    }
    
    # In the second example, the patterns that may match have
    # common prefixes, and more than one pattern may match. We
    # add patterns to the set in a "more specific" to "less
    # specific" order, and we choose the most specific pattern
    # that matches, by specifying the first matching pattern in
    # the set.
    
    sub vcl_init {
        # With anchor=start, we specify matching prefixes.
        new matcher = re2.set(anchor=start);
        matcher.add("/foo/bar/baz/quux", "/baz/quux");
        matcher.add("/foo/bar/baz", "/baz/quux/foo");
        matcher.add("/foo/bar", "/baz/quux/foo/bar");
        matcher.add("/foo", "/baz");
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            # Select the first matching pattern
            return(synth(1301, matcher.string(select=FIRST)));
        }
    }
    
    # vcl_synth is implemented as shown above

#### BACKEND xset.backend(INT n, ENUM select)

    BACKEND xset.backend(
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns the backend associated with the nth pattern added to the set, or
with the pattern in the set that matched in the most recent call to
`.match()` in the same task scope (client or backend context).

The rules for selecting a pattern from the set and its associated
backend based on `n` and `select` are the same as described above for
`.string()`.

`.backend()` invokes VCL failure under the same conditions described for
`.string()` above -- `n` and `select` are invalid, or no backend was
associated with the selected pattern with the `.add()` method (see
[ERRORS](#errors)).

Example:

    # Choose a backend based on the URL prefix.
    
    # In this example, assume that backends b1 through b4
    # have been defined.
    
    sub vcl_init {
        # Use anchor=start to match prefixes.
        # The prefixes are unique, so exactly one will match.
        new matcher = re2.set(anchor=start);
        matcher.add("/foo", backend=b1);
        matcher.add("/bar", backend=b2);
        matcher.add("/baz", backend=b3);
        matcher.add("/quux", backend=b4);
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            # Confirm that there was exactly one match
            if (matcher.nmatches() != 1) {
                return(fail);
            }
            # Set the backend hint to the backend associated
            # with the matching pattern.
            set req.backend_hint = matcher.backend();
        }
    }

#### INT xset.integer(INT n, ENUM select)

    INT xset.integer(
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns the integer associated with the nth pattern added to the set, or
with the pattern in the set that matched in the most recent call to
`.match()` in the same task scope.

The rules for selecting a pattern from the set and its associated
integer based on `n` and `select` are the same as described above for
`.string()`.

`.integer()` invokes VCL failure under the same error conditions
described for `.string()` above -- `n` and `select` are invalid, or no
integer was associated with the selected pattern with the `.add()`
method (see [ERRORS](#errors)).

Example:

    # Generate redirect responses based on the Host header. In the
    # example, subdomains are removed in the new Location, and the
    # associated integer is used to set the redirect status code.
    
    sub vcl_init {
        # No more than one pattern can match the same string. So it
        # is safe to call .integer() with default select=UNIQUE in
        # vcl_recv below (no risk of VCL failure).
        new redir = re2.set(anchor=both);
        redir.add("www\.[^.]+\.foo\.com", integer=301, string="www.foo.com");
        redir.add("www\.[^.]+\.bar\.com", integer=302, string="www.bar.com");
        redir.add("www\.[^.]+\.baz\.com", integer=303, string="www.baz.com");
        redir.add("www\.[^.]+\.quux\.com", integer=307, string="www.quux.com");
        redir.compile();
    }
    
    sub vcl_recv {
        if (redir.match(req.http.Host)) {
            # Construct a Location header that will be used in the
            # synthetic redirect response.
            set req.http.Location = "http://" + redir.string() + req.url;
    
        # Set the response status from the associated integer.
        return( synth(redir.integer()) );
        }
    }
    
    sub vcl_synth {
        if (resp.status >= 301 && resp.status <= 307) {
            # We come here from the synth return for the redirect
        # response. The status code was set from .integer().
            set resp.http.Location = req.http.Location;
        return(deliver);
        }
    }

#### STRING xset.sub(STRING text, STRING rewrite, STRING fallback, INT n, ENUM select)

    STRING xset.sub(
          STRING text,
          STRING rewrite,
          STRING fallback="**SUB METHOD FAILED**",
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns the result of the method call `.sub(text, rewrite, fallback)`,
as documented above for the `regex` interface, invoked on the nth
pattern added to the set, or on the pattern in the set that matched in
the most recent call to `.match()` in the same task scope.

`.sub()` requires that the pattern it identifies was saved as an
internal `regex` object, by setting `save` to true when it was added
with the `.add()` method.

The associated pattern is determined by `n` and `select` according to
the rules given above. If an internal `regex` object was saved for that
pattern, then the result of the `.sub()` method invoked on that object
is returned.

`.sub()` invokes VCL failkure (see [ERRORS](#errors)) if:

  - The values of `n` and `select` are invalid, according to the rules
    given above.
  - `save` was false in the `.add()` method for the pattern identified
    by `n` and `select`; that is, no internal `regex` object was saved
    on which the `.sub()` method could have been invoked.
  - The `.sub()` method invoked on the `regex` object fails for any of
    the reasons described for `regex.sub()`.

Examples:

    # Generate synthethic redirect responses on URLs that match a set of
    # patterns, rewriting the URL according to the matched pattern.
    
    # In this example, we set the new URL in the redirect location to
    # the path that comes after the prefix of the original req.url.
    sub vcl_init {
        new matcher = re2.set(anchor=start);
        matcher.add("/foo/(.*)", save=true);
        matcher.add("/bar/(.*)", save=true);
        matcher.add("/baz/(.*)", save=true);
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            if (matcher.nmatches() != 1) {
                return(fail);
            }
            return(synth(1301));
        }
    }
    
    sub vcl_synth {
        if (resp.status == 1301) {
            # matcher.sub() rewrites the URL to the subpath after the
        # original prefix.
            set resp.http.Location
                = "http://www.otherdomain.org" + matcher.sub(req.url, "/\1");
            return(deliver);
        }
    }

#### STRING xset.suball(STRING text, STRING rewrite, STRING fallback, INT n, ENUM select)

    STRING xset.suball(
          STRING text,
          STRING rewrite,
          STRING fallback="**SUBALL METHOD FAILED**",
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Like the `.sub()` method, this returns the result of calling
`.suball(text, rewrite, fallback)` from the regex interface on the nth
pattern added to the set, or the pattern that most recently matched in a
`.match()` call.

`.suball()` is subject to the same conditions as the `.sub()` method:

  - The pattern to which it is applied is identified by `n` and `select`
    according to the rules given above.
  - It fails if:
      - The pattern that it identifies was not saved with
        `.add(save=true)`.
      - The values of `n` or `select` are invalid.
      - The `.suball()` method invoked on the saved `regex` object
        fails.

Example:

    # In any URL that matches one of the words given below, replace all
    # occurrences of the matching word with "quux" (for example to
    # rewrite path components or elements of query strings).
    sub vcl_init {
        new matcher = re2.set();
        matcher.add("\bfoo\b", save=true);
        matcher.add("\bbar\b", save=true);
        matcher.add("\bbaz\b", save=true);
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            if (matcher.nmatches() != 1) {
                return(fail);
            }
            set req.url = matcher.suball(req.url, "quux");
        }
    }

#### STRING xset.extract(STRING text, STRING rewrite, STRING fallback, INT n, ENUM select)

    STRING xset.extract(
          STRING text,
          STRING rewrite,
          STRING fallback="**EXTRACT METHOD FAILED**",
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Like the `.sub()` and `.suball()` methods, this method returns the
result of calling `.extract(text, rewrite, fallback)` from the regex
interface on the nth pattern added to the set, or the pattern that most
recently matched in a `.match()` call.

`.extract()` is subject to the same conditions as the other rewrite
methods:

  - The pattern to which it is applied is identified by `n` and `select`
    according to the rules given above.
  - It fails if:
      - The pattern that it identifies was not saved with
        `.add(save=true)`.
      - The values of `n` or `select` are invalid.
      - The `.extract()` method invoked on the saved `regex` object
        fails.

Example:

    # Rewrite any URL that matches one of the patterns in the set
    # by exchanging the path components.
    sub vcl_init {
        new matcher = re2.set(anchor=both);
        matcher.add("/(foo)/(bar)/", save=true);
        matcher.add("/(bar)/(baz)/", save=true);
        matcher.add("/(baz)/(quux)/", save=true);
        matcher.compile();
    }
    
    sub vcl_recv {
        if (matcher.match(req.url)) {
            if (matcher.nmatches() != 1) {
                return(fail);
            }
            set req.url = matcher.extract(req.url, "/\2/\1/");
        }
    }

#### SUB xset.subroutine(INT n, ENUM select)

    SUB xset.subroutine(
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns the subroutine set by the `sub` parameter for the element of the
set indicated by `n` and `select`, according to the rules given above.
The subroutine may be invoked with VCL `call`.

**Note**: you must ensure that the subroutine may invoked legally in the
context in which it is called. This means that:

  - The subroutine may only refer to VCL elements that are legal in the
    invocation context. For example, if the subroutine only refers to
    headers in `req.http.*`, then it may be called in `vcl_recv`, but
    not if it refers to any header in `resp.http.*`. See `vcl-var(7)`
    for the specification of which VCL variables may be used in which
    contexts.
  - Recursive subroutine calls are not permitted in VCL. The subroutine
    invocation may not appear anywhere in its own call stack.

For standard subroutine invocations with `call`, the VCL compiler checks
these conditions and issues a compile-time error if either one is
violated. This is not possible with invocations using `.subroutine()`;
the error can only be determined at runtime. So it is advisable to test
the use of `.subroutine()` carefully before using it in production. You
can use the `.check_call()` method described below to determine if the
subroutine call is legal.

`.subroutine()` invokes VCL failure (See [ERRORS](#errors)) if:

  - The rules for `n` and `select` indicate failure.
  - No subroutine was set with the `sub` parameter in `.add()`.
  - The subroutine is invoked with `call`, but the call is not legal in
    the invocation context, for the reasons given above.

Example:

    # Due to the use of resp.http.*, this subroutine may only be invoked
    # in vcl_deliver or vcl_synth, as documented in vcl-var(7). Note
    # that subroutine definitions must appear before vcl_init to
    # permitted for the sub parameter in .add().
    sub resp_sub {
        set resp.http.Call-Me = "but only in deliver or synth";
    }
    
    sub vcl_init {
        new myset = re2.set();
        myset.add("/foo", sub=resp_sub);
        myset.add("/foo/bar", sub=some_other_sub);
        # ...
    }
    
    sub vcl_deliver {
        if (myset.match(req.url)) {
           call myset.subroutine(select=FIRST);
        }
    }

#### BOOL xset.check\_call(INT n, ENUM select)

    BOOL xset.check_call(
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns `true` iff the subroutine returned by `.subroutine()` for the
element of the set indicated by `n` and `select` may be invoked legally
in the current context. The conditions for legal invocation are
documented for `.subroutine()` above.

`.check_call()` never invokes VCL failure, but rather returns `false`
under conditions for which the use of `.subroutine()` would invoke VCL
failure. In that case, a message is emitted to the Vanrish log using the
`Notice` tag (the same message that would appear with the `VCL_Error`
tag if the subroutine were called).

`Notice` messages in the log produced by this VMOD are always prefixed
with the string `vmod_re2:`.

Example:

    # Assume that myset is declared as in the example above.
    sub vcl_deliver {
        if (myset.match(req.url)) {
           if (myset.check_call(select=FIRST)) {
              call myset.subroutine(select=FIRST);
           }
           else {
              call do_if_resp_sub_is_illegal;
           }
        }
    }

#### BOOL xset.saved(ENUM which, INT n, ENUM select)

    BOOL xset.saved(
          ENUM {REGEX, STR, BE, INT, SUB} which=REGEX,
          INT n=0,
          ENUM {FIRST, LAST, UNIQUE} select=UNIQUE
    )

Returns true if and only if an object of the type indicated by `which`
was saved at initialization time for the `nth` pattern added to the set,
or for the pattern indicated by `select` after the most recent
`.match()` call.

In other words, `.saved()` returns true:

  - for `which=REGEX` if the individual regex was saved with
    `.add(save=true)` for the indicated pattern
  - for `which=STR` if a string was stored with the `string` parameter
    in `.add()`
  - for `which=BE` if a backend was stored with the `backend` attribute.
  - for `which=INT` if an integer was stored with the `integer`
    attribute.
  - for `which=SUB` if an integer was stored with the `sub` attribute.

The default value of `which` is `REGEX`.

The pattern in the set is identified by `n` and `select` according to
the rules given above. `.saved()` invokes VCL failure if the values of
`n` or `select` are invalid (see [ERRORS](#errors)).

Example:

    sub vcl_init {
        new s = re2.set();
        s.add("1", save=true, string="1", backend=b1);
        s.add("2", save=true, string="2");
        s.add("3", save=true, backend=b3);
        s.add("4", save=true);
        s.add("5", string="5", backend=b5);
        s.add("6", string="6");
        s.add("7", backend=b7);
        s.add("8");
        s.compile();
    }
    
    # Then the following holds for this set:
    # s.saved(n=1) == true    # for any value of which
    # s.saved(which=REGEX, n=2) == true
    # s.saved(which=STR, n=2)   == true
    # s.saved(which=BE, n=2)    == false
    # s.saved(which=REGEX, n=3) == true
    # s.saved(which=STR, n=3)   == false
    # s.saved(which=BE, n=3)    == true
    # s.saved(which=REGEX, n=4) == true
    # s.saved(which=STR, n=4)   == false
    # s.saved(which=BE, n=4)    == false
    # s.saved(which=REGEX, n=5) == false
    # s.saved(which=STR, n=5)   == true
    # s.saved(which=BE, n=5)    == true
    # s.saved(which=REGEX, n=6) == false
    # s.saved(which=STR, n=6)   == true
    # s.saved(which=BE, n=6)    == false
    # s.saved(which=REGEX, n=7) == false
    # s.saved(which=STR, n=7)   == false
    # s.saved(which=BE, n=7)    == true
    # s.saved(n=8) == false   # for any value of which
    
    if (s.match("4")) {
       # The fourth pattern has been uniquely matched.
       # So in this context: s.saved() == true
       # Since save=true was used in .add() for the 4th pattern,
       # and which=REGEX by default.
    }

#### VOID xset.hdr\_filter(HTTP, BOOL whitelist=1)

Filters the headers in the HTTP object, which may be one of `req`,
`resp`, `bereq`, or `beresp`. In other words, filter the headers in the
client or backend request or response.

If `whitelist` is `true`, then headers that match one of the patterns in
the set are retained, and all other headers are removed. Otherwise,
headers that match a pattern in the set are removed, and all others are
retained. By default, `whitelist` is `true`.

Example:

    sub vcl_init {
      # Header whitelist
      new white = re2.set(anchor=start);
      white.add("Foo:");
      white.add("Bar:");
      white.add("Baz: baz$");
      white.compile();
    
      # Header blacklist
      new black = re2.set(anchor=start);
      black.add("Chaotic:");
      black.add("Evil:");
      black.add("Wicked: wicked$");
      black.compile();
    }
    
    sub vcl_recv {
      # Filter the client request header with the whitelist.
      # Headers that do not match any pattern in the set are removed.
      white.hdr_filter(req);
    }
    
    sub vcl_deliver {
      # Filter the client response header with the blacklist.
      # Headers that match any pattern in the set are removed.
      black.hdr_filter(resp, false);
    }

#### STRING quotemeta(STRING)

Returns a copy of the argument string with all regex metacharacters
escaped via backslash. When the returned string is used as a regular
expression, it will exactly match the original string, regardless of any
special characters. This function has a purpose similar to a `\Q..\E`
sequence within a regex, or the `literal=true` setting in a regex
constructor.

The function invokes VCL failure if there is insufficient workspace for
the return string (see [ERRORS](#errors)).

Example:

    # The following are always true:
    re2.quotemeta("1.5-2.0?") == "1\.5\-2\.0\?"
    re2.match(re2.quotemeta("1.5-2.0?"), "1.5-2.0?")

#### STRING version()

Return the version string for this VMOD.

Example:

    std.log("Using VMOD re2 version: " + re2.version());

### ERRORS

Functions and methods of the VMOD may invoke VCL failure under
unrecoverable error conditions. The effects of VCL failure depend on the
VCL subroutine in which it takes place:

  - If invoked during `vcl_init`, then the VCL load fails, and an error
    message is returned over the CLI (reported by `varnishadm(1)`).
  - If invoked during any other subroutine besides `vcl_synth`, then an
    error message is recorded in the log with the `VCL_Error` tag,
    further processing is aborted immediately, and a response with
    status 503 (Service Not Available) is returned with the reason
    string "VCL failed".
  - If invoked during `vcl_synth`, then further processing is aborted,
    the error message is logged with `VCL_Error`, and the client
    connection is immediately closed -- the client receives no response.

Errors that lead to VCL failure include:

  - Any regex compilation failure.
  - Out of workspace errors (see [LIMITATIONS](#limitations)).
  - Failures reported by the RE2 library for: matches, backrefs,
    namedrefs, the rewrite operations (sub, suball and extract), the
    `.cost()` function or method, and the `.quotemeta()` function. The
    VMOD detects most common errors that would lead to library errors,
    and invokes VCL failure in such cases without calling the library.
    But library errors may happen under conditions such as out of
    memory.
  - Functions and methods that require a previous successful match when
    there was no prior match, or when the previous match was
    unsuccessful. These include backrefs, namedrefs, and the data
    retrieval methods for set objects.
  - Any of the following parameters are undefined, for example when set
    from an unset header: fallbacks; patterns for the regex functions
    (which are compiled at runtime); the text and rewrite parameters for
    rewrite operations; the name parameter for namedrefs.
  - The name parameter for namedrefs is the empty string.
  - Backref number is out of range (greater than the number of backrefs
    in the pattern).
  - Backref or namedref attempted when the `never_capture` option was
    set to `true` for the pattern.
  - For set objects:
      - Numeric index (parameter `n`) is out of range (greater than the
        number of patterns in the set).
      - Use of `select=UNIQUE` after more than one pattern was matched.
        The `.nmatches()` can be used to check for this condition, to
        avoid VCL failure -- `UNIQUE` will fail in `.namtches()` \> 1.
      - Retrieval of data from a set (such as a string, backend etc) by
        numeric index (`n`) or "associatively" (after a match) when no
        such object was saved for the corresponding pattern. Use the
        `.saved()` and `.check_call()` methods to check for this.
      - Calling the subroutine returned by `.subrooutine()` may be
        illegal, if it is not permitted in the subroutine from which it
        is called, or if it would lead to recursive calls. Use the
        `.check_call()` method to check for this.

### REQUIREMENTS

The VMOD requires Varnish since version 6.6, or the master branch. See
the source repository for versions of the VMOD that are compatible with
other Varnish versions.

It requires the RE2 library, and has been tested against RE2 versions
since 2015-06-01 (through 2021-04-01 at the time of writing).

If the VMOD is built against versions of RE2 since 2017-12-01, it uses a
version of the set match operation that reports out-of-memory conditions
during a match. (Versions of RE2 since June 2019 no longer have this
error, but nevertheless the different internal call is used for set
matches.) In that case, the VMOD is not compatible with earlier versions
of RE2. This is only a problem if the runtime version of the library
differs from the version against which the VMOD was built. If you
encounter this error, consider re-building the VMOD against the runtime
version of RE2, or installing a newer version of RE2.

### INSTALLATION

See [INSTALL.rst](INSTALL.rst) in the source repository.

### LIMITATIONS

The VMOD allocates Varnish workspace for captured groups and rewritten
strings. If operations fail with "insufficient workspace" error messages
in the Varnish log (with the `VCL_Error` tag), increase the varnishd
runtime parameters `workspace_client` and/or `workspace_backend`.

The RE2 documentation states that successful matches are slowed quite a
bit when they also capture substrings. There is also additional overhead
from the VMOD, unless the `never_capture` flag is true, to manage data
about captured groups in the workspace. This overhead is incurred even
if there are no capturing expressions in a pattern, since it is always
possible to call `backref(0)` to obtain the matched portion of a string.

So if you are using a pattern only to match against strings, and never
to capture subexpressions, consider setting the `never_capture` option
to true, to eliminate the extra work for both RE2 and the VMOD.

### AUTHOR

  - Geoffrey Simmons \<<geoff@uplex.de>\>

UPLEX Nils Goroll Systemoptimierung

### SEE ALSO

  - varnishd(1)
  - vcl(7)
  - VMOD source repository:
    <https://code.uplex.de/uplex-varnish/libvmod-re2>
      - Gitlab mirror: <https://gitlab.com/uplex/varnish/libvmod-re2>
  - RE2 git repo: <https://github.com/google/re2>
  - RE2 syntax: <https://github.com/google/re2/wiki/Syntax>
  - "Implementing Regular Expressions": <https://swtch.com/~rsc/regexp/>
      - Series of articles motivating the design of RE2, with discussion
        of how RE2 compares with PCRE

### COPYRIGHT

    Copyright (c) 2016-2018 UPLEX Nils Goroll Systemoptimierung
    All rights reserved
    
    Author: Geoffrey Simmons <geoffrey.simmons@uplex.de>
    
    See LICENSE
