Provided by: libppix-regexp-perl_0.088-1_all 
NAME
PPIx::Regexp - Represent a regular expression of some sort
SYNOPSIS
use PPIx::Regexp;
use PPIx::Regexp::Dumper;
my $re = PPIx::Regexp->new( 'qr{foo}smx' );
PPIx::Regexp::Dumper->new( $re )
->print();
DEPRECATION NOTICE
The "postderef" argument to new() is retracted, and postfix dereferences are always be recognized.
Starting with version 0.074_01, the first use of this argument warned. With version 0.079_01, all uses
warned. With version 0.080_01, all uses became fatal. With version 0.084_01, all mention of this argument
was removed, except for this notice.
INHERITANCE
"PPIx::Regexp" is a PPIx::Regexp::Node.
"PPIx::Regexp" has no descendants.
DESCRIPTION
The purpose of the PPIx-Regexp package is to parse regular expressions in a manner similar to the way the
PPI package parses Perl. This class forms the root of the parse tree, playing a role similar to
PPI::Document.
This package shares with PPI the property of being round-trip safe. That is,
my $expr = 's/ ( \d+ ) ( \D+ ) /$2$1/smxg';
my $re = PPIx::Regexp->new( $expr );
print $re->content() eq $expr ? "yes\n" : "no\n"
should print 'yes' for any valid regular expression.
Navigation is similar to that provided by PPI. That is to say, things like "children", "find_first",
"snext_sibling" and so on all work pretty much the same way as in PPI.
The class hierarchy is also similar to PPI. Except for some utility classes (the dumper, the lexer, and
the tokenizer) all classes are descended from PPIx::Regexp::Element, which provides basic navigation.
Tokens are descended from PPIx::Regexp::Token, which provides content. All containers are descended from
PPIx::Regexp::Node, which provides for children, and all structure elements are descended from
PPIx::Regexp::Structure, which provides beginning and ending delimiters, and a type.
There are two features of PPI that this package does not provide - mutability and operator overloading.
There are no plans for serious mutability, though something like PPI's "prune" functionality might be
considered. Similarly there are no plans for operator overloading, which appears to the author to
represent a performance hit for little tangible gain.
NOTICE
The author will attempt to preserve the documented interface, but if the interface needs to change to
correct some egregiously bad design or implementation decision, then it will change. Any incompatible
changes will go through a deprecation cycle.
The goal of this package is to parse well-formed regular expressions correctly. A secondary goal is not
to blow up on ill-formed regular expressions. The correct identification and characterization of ill-
formed regular expressions is not a goal of this package, nor is the consistent parsing of ill-formed
regular expressions from release to release.
This policy attempts to track features in development releases as well as public releases. However,
features added in a development release and then removed before the next production release will not be
tracked, and any functionality relating to such features will be removed. The issue here is the potential
re-use (with different semantics) of syntax that did not make it into the production release.
From time to time the Perl regular expression engine changes in ways that change the parse of a given
regular expression. When these changes occur, "PPIx::Regexp" will be changed to produce the more modern
parse. Known examples of this include:
$( no longer interpolates as of Perl 5.005, per "perl5005delta".
Newer Perls seem to parse this as "qr{$}" (i.e. an end-of-string or newline assertion) followed by an
open parenthesis, and that is what "PPIx::Regexp" does.
$) and $| also seem to parse as the "$" assertion
followed by the relevant meta-character, though I have no documentation reference for this.
"@+" and "@-" no longer interpolate as of Perl 5.9.4
per "perl594delta". Subsequent Perls treat "@+" as a quantified literal and "@-" as two literals, and
that is what "PPIx::Regexp" does. Note that subscripted references to these arrays do interpolate,
and are so parsed by "PPIx::Regexp".
Only space and horizontal tab are whitespace as of Perl 5.23.4
when inside a bracketed character class inside an extended bracketed character class, per
"perl5234delta". Formerly any white space character parsed as whitespace. This change in
"PPIx::Regexp" will be reverted if the change in Perl does not make it into Perl 5.24.0.
Unescaped literal left curly brackets
These are being removed in positions where quantifiers are legal, so that they can be used for new
functionality. Some of them are gone in 5.25.1, others will be removed in a future version of Perl.
In situations where they have been removed, perl_version_removed() will return the version in which
they were removed. When the new functionality appears, the parse produced by this software will
reflect the new functionality.
NOTE that the situation with a literal left curly after a literal character is complicated. It was
made an error in Perl 5.25.1, and remained so through all 5.26 releases, but became a warning again
in 5.27.1 due to its use in GNU Autoconf. Whether it will ever become illegal again is not clear to
me based on the contents of perl5271delta. At the moment perl_version_removed() returns "undef", but
obviously that is not the whole story, and methods accepts_perl() and requirements_for_perl() were
introduced to deal with this complication.
"\o{...}"
is parsed as the octal equivalent of "\x{...}". This is its meaning as of perl 5.13.2. Before 5.13.2
it was simply literal 'o' and so on.
"x{,3}"
(with first count omitted) is allowed as a quantifier as of Perl 5.33.6. The previous parse made
this all literals.
"x{ 0 , 3 }"
(with spaces inside but adjacent to curly brackets, or around the comma if any) is allowed as a
quantifier as of Perl 5.33.6. The previous parse made this all literals.
There are very probably other examples of this. When they come to light they will be documented as
producing the modern parse, and the code modified to produce this parse if necessary.
METHODS
This class provides the following public methods. Methods not documented here are private, and
unsupported in the sense that the author reserves the right to change or remove them without notice.
new
my $re = PPIx::Regexp->new('/foo/');
This method instantiates a "PPIx::Regexp" object from a string, a PPI::Token::QuoteLike::Regexp, a
PPI::Token::Regexp::Match, or a PPI::Token::Regexp::Substitute. Honestly, any PPI::Element will work,
but only the three Regexp classes mentioned previously are likely to do anything useful.
Whatever form the argument takes, it is assumed to consist entirely of a valid match, substitution, or
"qr<>" string.
Optionally you can pass one or more name/value pairs after the regular expression. The possible options
are:
default_modifiers array_reference
This option specifies a reference to an array of default modifiers to apply to the regular expression
being parsed. Each modifier is specified as a string. Any actual modifiers found supersede the
defaults.
When applying the defaults, '?' and '/' are completely ignored, and '^' is ignored unless it occurs
at the beginning of the modifier. The first dash ('-') causes subsequent modifiers to be negated.
So, for example, if you wish to produce a "PPIx::Regexp" object representing the regular expression
in
use re '/smx';
{
no re '/x';
m/ foo /;
}
you would (after some help from PPI in finding the relevant statements), do something like
my $re = PPIx::Regexp->new( 'm/ foo /',
default_modifiers => [ '/smx', '-/x' ] );
encoding name
This option specifies the encoding of the regular expression. This is passed to the tokenizer, which
will "decode" the regular expression string before it tokenizes it. For example:
my $re = PPIx::Regexp->new( '/foo/',
encoding => 'iso-8859-1',
);
index_locations Boolean
This Boolean option specifies whether the locations of the elements in the regular expression should
be indexed.
If unspecified or specified as "undef" a default value is used. This default is true if the argument
is a PPI::Element or the "location" option was specified. Otherwise the default is false.
location array_reference
This option specifies the location of the new object in the document from which it was created. It is
a reference to a five-element array compatible with that returned by the "location()" method of
PPI::Element.
If not specified, the location of the original string is used if it was specified as a PPI::Element.
If no location can be determined, the various "location()" methods will return "undef".
postderef Boolean
THIS ARGUMENT IS DEPRECATED. See DEPRECATION NOTICE above for the details.
This option is passed on to the tokenizer, where it specifies whether postfix dereferences are
recognized in interpolations and code. This experimental feature was introduced in Perl 5.19.5.
As of version 0.074_01, the default is true. Through release 0.074, the default was the value of
$PPIx::Regexp::Tokenizer::DEFAULT_POSTDEREF, which was true. When originally introduced this was
false, but was documented as becoming true when and if postfix dereferencing became mainstream. The
intent to mainstream was announced with Perl 5.23.1, and became official (so to speak) with Perl
5.24.0, so the default became true with PPIx::Regexp 0.049_01.
Note that if PPI starts unconditionally recognizing postfix dereferences, this argument will
immediately become ignored, and will be put through a deprecation cycle and removed.
strict Boolean
This option is passed on to the tokenizer and lexer, where it specifies whether the parse should
assume "use re 'strict'" is in effect.
The 'strict' pragma was introduced in Perl 5.22, and its documentation says that it is experimental,
and that there is no commitment to backward compatibility. The same applies to the parse produced
when this option is asserted. Also, the usual caveat applies: if "use re 'strict'" ends up being
retracted, this option and all related functionality will be also.
Given the nature of "use re 'strict'", you should expect that if you assert this option, regular
expressions that previously parsed without error might no longer do so. If an element ends up being
declared an error because this option is set, its "perl_version_introduced()" will be the Perl
version at which "use re 'strict'" started rejecting these elements.
The default is false.
trace number
If greater than zero, this option causes trace output from the parse. The author reserves the right
to change or eliminate this without notice.
Passing optional input other than the above is not an error, but neither is it supported.
new_from_cache
This static method wraps "new" in a caching mechanism. Only one object will be generated for a given
PPI::Element, no matter how many times this method is called. Calls after the first for a given
PPI::Element simply return the same "PPIx::Regexp" object.
When the "PPIx::Regexp" object is returned from cache, the values of the optional arguments are ignored.
Calls to this method with the regular expression in a string rather than a PPI::Element will not be
cached.
Caveat: This method is provided for code like Perl::Critic which might instantiate the same object
multiple times. The cache will persist until "flush_cache" is called.
flush_cache
$re->flush_cache(); # Remove $re from cache
PPIx::Regexp->flush_cache(); # Empty the cache
This method flushes the cache used by "new_from_cache". If called as a static method with no arguments,
the entire cache is emptied. Otherwise any objects specified are removed from the cache.
capture_names
foreach my $name ( $re->capture_names() ) {
print "Capture name '$name'\n";
}
This convenience method returns the capture names found in the regular expression.
This method is equivalent to
$self->regular_expression()->capture_names();
except that if "$self->regular_expression()" returns "undef" (meaning that something went terribly wrong
with the parse) this method will simply return.
delimiters
print join("\t", PPIx::Regexp->new('s/foo/bar/')->delimiters());
# prints '// //'
When called in list context, this method returns either one or two strings, depending on whether the
parsed expression has a replacement string. In the case of non-bracketed substitutions, the start
delimiter of the replacement string is considered to be the same as its finish delimiter, as illustrated
by the above example.
When called in scalar context, you get the delimiters of the regular expression; that is, element 0 of
the array that is returned in list context.
Optionally, you can pass an index value and the corresponding delimiters will be returned; index 0
represents the regular expression's delimiters, and index 1 represents the replacement string's
delimiters, which may be undef. For example,
print PPIx::Regexp->new('s{foo}<bar>')->delimiters(1);
# prints '<>'
If the object was not initialized with a valid regexp of some sort, the results of this method are
undefined.
errstr
This static method returns the error string from the most recent attempt to instantiate a "PPIx::Regexp".
It will be "undef" if the most recent attempt succeeded.
extract_regexps
my $doc = PPI::Document->new( $path );
$doc->index_locations();
my @res = PPIx::Regexp->extract_regexps( $doc )
This convenience (well, sort-of) static method takes as its argument a PPI::Document object and returns
"PPIx::Regexp" objects corresponding to all regular expressions found in it, in the order in which they
occur in the document. You will need to keep a reference to the original PPI::Document object if you wish
to be able to recover the original PPI::Element objects via the PPIx::Regexp source() method.
failures
print "There were ", $re->failures(), " parse failures\n";
This method returns the number of parse failures. This is a count of the number of unknown tokens plus
the number of unterminated structures plus the number of unmatched right brackets of any sort.
max_capture_number
print "Highest used capture number ",
$re->max_capture_number(), "\n";
This convenience method returns the highest capture number used by the regular expression. If there are
no captures, the return will be 0.
This method is equivalent to
$self->regular_expression()->max_capture_number();
except that if "$self->regular_expression()" returns "undef" (meaning that something went terribly wrong
with the parse) this method will too.
modifier
my $re = PPIx::Regexp->new( 's/(foo)/${1}bar/smx' );
print $re->modifier()->content(), "\n";
# prints 'smx'.
This method retrieves the modifier of the object. This comes from the end of the initializing string or
object and will be a PPIx::Regexp::Token::Modifier.
Note that this object represents the actual modifiers present on the regexp, and does not take into
account any that may have been applied by default (i.e. via the "default_modifiers" argument to "new()").
For something that takes account of default modifiers, see modifier_asserted(), below.
In the event of a parse failure, there may not be a modifier present, in which case nothing is returned.
modifier_asserted
my $re = PPIx::Regexp->new( '/ . /',
default_modifiers => [ 'smx' ] );
print $re->modifier_asserted( 'x' ) ? "yes\n" : "no\n";
# prints 'yes'.
This method returns true if the given modifier is asserted for the regexp, whether explicitly or by the
modifiers passed in the "default_modifiers" argument.
Starting with version 0.036_01, if the argument is a single-character modifier followed by an asterisk
(intended as a wild card character), the return is the number of times that modifier appears. In this
case an exception will be thrown if you specify a multi-character modifier (e.g. 'ee*'), or if you
specify one of the match semantics modifiers (e.g. 'a*').
regular_expression
my $re = PPIx::Regexp->new( 's/(foo)/${1}bar/smx' );
print $re->regular_expression()->content(), "\n";
# prints '/(foo)/'.
This method returns that portion of the object which actually represents a regular expression.
replacement
my $re = PPIx::Regexp->new( 's/(foo)/${1}bar/smx' );
print $re->replacement()->content(), "\n";
# prints '${1}bar/'.
This method returns that portion of the object which represents the replacement string. This will be
"undef" unless the regular expression actually has a replacement string. Delimiters will be included, but
there will be no beginning delimiter unless the regular expression was bracketed.
source
my $source = $re->source();
This method returns the object or string that was used to instantiate the object.
type
my $re = PPIx::Regexp->new( 's/(foo)/${1}bar/smx' );
print $re->type()->content(), "\n";
# prints 's'.
This method retrieves the type of the object. This comes from the beginning of the initializing string or
object, and will be a PPIx::Regexp::Token::Structure whose "content" is one of 's', 'm', 'qr', or ''.
RESTRICTIONS
By the nature of this module, it is never going to get everything right. Many of the known problem areas
involve interpolations one way or another.
Ambiguous Syntax
Perl's regular expressions contain cases where the syntax is ambiguous. A particularly egregious example
is an interpolation followed by square or curly brackets, for example $foo[...]. There is nothing in the
syntax to say whether the programmer wanted to interpolate an element of array @foo, or whether he wanted
to interpolate scalar $foo, and then follow that interpolation by a character class.
The perlop documentation notes that in this case what Perl does is to guess. That is, it employs various
heuristics on the code to try to figure out what the programmer wanted. These heuristics are documented
as being undocumented (!) and subject to change without notice. As an example of the problems even perl
faces in parsing Perl, see <https://github.com/perl/perl5/issues/16478>.
Given this situation, this module's chances of duplicating every Perl version's interpretation of every
regular expression are pretty much nil. What it does now is to assume that square brackets containing
only an integer or an interpolation represent a subscript; otherwise they represent a character class.
Similarly, curly brackets containing only a bareword or an interpolation are a subscript; otherwise they
represent a quantifier.
Changes in Syntax
Sometimes the introduction of new syntax changes the way a regular expression is parsed. For example, the
"\v" character class was introduced in Perl 5.9.5. But it did not represent a syntax error prior to that
version of Perl, it was simply parsed as "v". So
$ perl -le 'print "v" =~ m/\v/ ? "yes" : "no"'
prints "yes" under Perl 5.8.9, but "no" under 5.10.0. "PPIx::Regexp" generally assumes the more modern
parse in cases like this.
Equivocation
Very occasionally, a construction will be removed and then added back -- and then, conceivably, removed
again. In this case, the plan is for perl_version_introduced() to return the earliest version in which
the construction appeared, and perl_version_removed() to return the version after the last version in
which it appeared (whether production or development), or "undef" if it is in the highest-numbered Perl.
The constructions involved in this are:
Un-escaped literal left curly after literal
That is, something like "qr<x{>".
This was made an error in 5.25.1, and it was an error in 5.26.0. But it became a warning again in
5.27.1. The perl5271delta says it was re-instated because the changes broke GNU Autoconf, and the warning
message says it will be removed in Perl 5.30.
Accordingly, perl_version_introduced() returns 5.0. At the moment perl_version_removed() returns
'5.025001'. But if it is present with or without warning in 5.28, perl_version_removed() will become
"undef". If you need finer resolution than this, see PPIx::Regexp::Element methods
l<accepts_perl()|PPIx::Regexp::Element/accepts_perl> and
l<requirements_for_perl()|PPIx::Regexp::Element/requirements_for_perl>
Static Parsing
It is well known that Perl can not be statically parsed. That is, you can not completely parse a piece of
Perl code without executing that same code.
Nevertheless, this class is trying to statically parse regular expressions. The main problem with this is
that there is no way to know what is being interpolated into the regular expression by an interpolated
variable. This is a problem because the interpolated value can change the interpretation of adjacent
elements.
This module deals with this by making assumptions about what is in an interpolated variable. These
assumptions will not be enumerated here, but in general the principal is to assume the interpolated value
does not change the interpretation of the regular expression. For example,
my $foo = 'a-z]';
my $re = qr{[$foo};
is fine with the Perl interpreter, but will confuse the dickens out of this module. Similarly and more
usefully, something like
my $mods = 'i';
my $re = qr{(?$mods:foo)};
or maybe
my $mods = 'i';
my $re = qr{(?$mods)$foo};
probably sets a modifier of some sort, and that is how this module interprets it. If the interpolation is
not about modifiers, this module will get it wrong. Another such semi-benign example is
my $foo = $] >= 5.010 ? '?<foo>' : '';
my $re = qr{($foo\w+)};
which will parse, but this module will never realize that it might be looking at a named capture.
Non-Standard Syntax
There are modules out there that alter the syntax of Perl. If the syntax of a regular expression is
altered, this module has no way to understand that it has been altered, much less to adapt to the
alteration. The following modules are known to cause problems:
Acme::PerlML, which renders Perl as XML.
"Data::PostfixDeref", which causes Perl to interpret suffixed empty brackets as dereferencing the thing
they suffix. This module by Ben Morrow ("BMORROW") appears to have been retracted.
Filter::Trigraph, which recognizes ANSI C trigraphs, allowing Perl to be written in the ISO 646 character
set.
Perl6::Pugs. Enough said.
Perl6::Rules, which back-ports some of the Perl 6 regular expression syntax to Perl 5.
Regexp::Extended, which extends regular expressions in various ways, some of which seem to conflict with
Perl 5.010.
SEE ALSO
Regexp::Parsertron, which uses Marpa::R2 to parse the regexp, and Tree for navigation. Unlike
"PPIx::Regexp|PPIx::Regexp", Regexp::Parsertron supports modification of the parse tree.
Regexp::Parser, which parses a bare regular expression (without enclosing "qr{}", "m//", or whatever) and
uses a different navigation model. After a long hiatus, this module has been adopted, and is again
supported.
YAPE::Regex, which provides the parse tree, and has a mechanism to subclass the various element classes
for customization. The most-recent release is 2011, but the CPAN testers results are still all green.
Companion module YAPE::Regex::Explain says what the various pieces of a regex do, though constructs added
in perl 5.10 and later are not supported. I have no idea how I missed this when I originally went looking
for "Regexp" parsers.
PPR, which recognizes Perl of all sorts, including regular expressions, but does not actually provide a
parse of the recognized constructs.
SUPPORT
Support is by the author. Please file bug reports at
<https://rt.cpan.org/Public/Dist/Display.html?Name=PPIx-Regexp>,
<https://github.com/trwyant/perl-PPIx-Regexp/issues>, or in electronic mail to the author.
AUTHOR
Thomas R. Wyant, III wyant at cpan dot org
COPYRIGHT AND LICENSE
Copyright (C) 2009-2023 by Thomas R. Wyant, III
This program is free software; you can redistribute it and/or modify it under the same terms as Perl
5.10.0. For more details, see the full text of the licenses in the directory LICENSES.
This program is distributed in the hope that it will be useful, but without any warranty; without even
the implied warranty of merchantability or fitness for a particular purpose.
perl v5.36.0 2023-03-05 PPIx::Regexp(3pm)