re(3perl) Perl Programmers Reference Guide re(3perl)
NAME
re - Perl pragma to alter regular expression behaviour
SYNOPSIS
use re 'taint';
($x) = ($^X =~ /^(.*)$/s); # $x is tainted here
$pat = '(?{ $foo = 1 })';
use re 'eval';
/foo${pat}bar/; # won't fail (when not under -T
# switch)
{
no re 'taint'; # the default
($x) = ($^X =~ /^(.*)$/s); # $x is not tainted here
no re 'eval'; # the default
/foo${pat}bar/; # disallowed (with or without -T
# switch)
}
use re 'strict'; # Raise warnings for more conditions
use re '/ix';
"FOO" =~ / foo /; # /ix implied
no re '/x';
"FOO" =~ /foo/; # just /i implied
use re 'debug'; # output debugging info during
/^(.*)$/s; # compile and run time
use re 'debugcolor'; # same as 'debug', but with colored
# output
...
use re qw(Debug All); # Same as "use re 'debug'", but you
# can use "Debug" with things other
# than 'All'
use re qw(Debug More); # 'All' plus output more details
no re qw(Debug ALL); # Turn on (almost) all re debugging
# in this scope
use re qw(is_regexp regexp_pattern); # import utility functions
my ($pat,$mods)=regexp_pattern(qr/foo/i);
if (is_regexp($obj)) {
print "Got regexp: ",
scalar regexp_pattern($obj); # just as perl would stringify
} # it but no hassle with blessed
# re's.
(We use $^X in these examples because it's tainted by default.)
DESCRIPTION
'taint' mode
When "use re 'taint'" is in effect, and a tainted string is the target
of a regexp, the regexp memories (or values returned by the m// operator
in list context) are tainted. This feature is useful when regexp
operations on tainted data aren't meant to extract safe substrings, but
to perform other transformations.
'eval' mode
When "use re 'eval'" is in effect, a regexp is allowed to contain "(?{
... })" zero-width assertions and "(??{ ... })" postponed subexpressions
that are derived from variable interpolation, rather than appearing
literally within the regexp. That is normally disallowed, since it is a
potential security risk. Note that this pragma is ignored when the
regular expression is obtained from tainted data, i.e. evaluation is
always disallowed with tainted regular expressions. See "(?{ code })"
in perlre and "(??{ code })" in perlre.
For the purpose of this pragma, interpolation of precompiled regular
expressions (i.e., the result of "qr//") is not considered variable
interpolation. Thus:
/foo${pat}bar/
is allowed if $pat is a precompiled regular expression, even if $pat
contains "(?{ ... })" assertions or "(??{ ... })" subexpressions.
'strict' mode
Note that this is an experimental feature which may be changed or
removed in a future Perl release.
When "use re 'strict'" is in effect, stricter checks are applied than
otherwise when compiling regular expressions patterns. These may cause
more warnings to be raised than otherwise, and more things to be fatal
instead of just warnings. The purpose of this is to find and report at
compile time some things, which may be legal, but have a reasonable
possibility of not being the programmer's actual intent. This
automatically turns on the "regexp" warnings category (if not already
on) within its scope.
As an example of something that is caught under ""strict'", but not
otherwise, is the pattern
qr/\xABC/
The "\x" construct without curly braces should be followed by exactly
two hex digits; this one is followed by three. This currently evaluates
as equivalent to
qr/\x{AB}C/
that is, the character whose code point value is 0xAB, followed by the
letter "C". But since "C" is a hex digit, there is a reasonable chance
that the intent was
qr/\x{ABC}/
that is the single character at 0xABC. Under 'strict' it is an error to
not follow "\x" with exactly two hex digits. When not under 'strict' a
warning is generated if there is only one hex digit, and no warning is
raised if there are more than two.
It is expected that what exactly 'strict' does will evolve over time as
we gain experience with it. This means that programs that compile under
it in today's Perl may not compile, or may have more or fewer warnings,
in future Perls. There is no backwards compatibility promises with
regards to it. Also there are already proposals for an alternate syntax
for enabling it. For these reasons, using it will raise a
"experimental::re_strict" class warning, unless that category is turned
off.
Note that if a pattern compiled within 'strict' is recompiled, say by
interpolating into another pattern, outside of 'strict', it is not
checked again for strictness. This is because if it works under strict
it must work under non-strict.
'/flags' mode
When "use re '/flags'" is specified, the given flags are automatically
added to every regular expression till the end of the lexical scope.
flags can be any combination of 'a', 'aa', 'd', 'i', 'l', 'm', 'n', 'p',
's', 'u', 'x', and/or 'xx'.
"no re '/flags'" will turn off the effect of "use re '/flags'" for the
given flags.
For example, if you want all your regular expressions to have /msxx on
by default, simply put
use re '/msxx';
at the top of your code.
The character set "/adul" flags cancel each other out. So, in this
example,
use re "/u";
"ss" =~ /\xdf/;
use re "/d";
"ss" =~ /\xdf/;
the second "use re" does an implicit "no re '/u'".
Similarly,
use re "/xx"; # Doubled-x
...
use re "/x"; # Single x from here on
...
Turning on one of the character set flags with "use re" takes precedence
over the "locale" pragma and the 'unicode_strings' "feature", for
regular expressions. Turning off one of these flags when it is active
reverts to the behaviour specified by whatever other pragmata are in
scope. For example:
use feature "unicode_strings";
no re "/u"; # does nothing
use re "/l";
no re "/l"; # reverts to unicode_strings behaviour
Default flags are applied to wherever a pattern is compiled with the
exception of the "/x" flag, which is not applied to patterns compiled
from string arguments to "split". Thus `use re "/x";` does not affect
the behaviour of "split " "" but does affect the behavior of "split /
/".
'debug' mode
When "use re 'debug'" is in effect, perl emits debugging messages when
compiling and using regular expressions. The output is the same as that
obtained by running a "-DDEBUGGING"-enabled perl interpreter with the
-Dr switch. It may be quite voluminous depending on the complexity of
the match. Using "debugcolor" instead of "debug" enables a form of
output that can be used to get a colorful display on terminals that
understand termcap color sequences. Set $ENV{PERL_RE_TC} to a comma-
separated list of "termcap" properties to use for highlighting strings
on/off, pre-point part on/off. See "Debugging Regular Expressions" in
perldebug for additional info.
NOTE that the exact format of the "debug" mode is NOT considered to be
an officially supported API of Perl. It is intended for debugging only
and may change as the core development team deems appropriate without
notice or deprecation in any release of Perl, major or minor. Any
documentation of the output is purely advisory.
As of 5.9.5 the directive "use re 'debug'" and its equivalents are
lexically scoped, as the other directives are. However they have both
compile-time and run-time effects.
See "Pragmatic Modules" in perlmodlib.
'Debug' mode
Similarly "use re 'Debug'" produces debugging output, the difference
being that it allows the fine tuning of what debugging output will be
emitted. Options are divided into three groups, those related to
compilation, those related to execution and those related to special
purposes.
NOTE that the options provided under the "Debug" mode and the exact
format of the output they create is NOT considered to be an officially
supported API of Perl. It is intended for debugging only and may change
as the core development team deems appropriate without notice or
deprecation in any release of Perl, major or minor. Any documentation of
the format or options available is advisory only and is subject to
change without notice.
The options are as follows:
Compile related options
COMPILE
Turns on all non-extra compile related debug options.
PARSE
Turns on debug output related to the process of parsing the
pattern.
OPTIMISE
Enables output related to the optimisation phase of compilation.
TRIEC
Detailed info about trie compilation.
DUMP
Dump the final program out after it is compiled and optimised.
FLAGS
Dump the flags associated with the program
TEST
Print output intended for testing the internals of the compile
process
Execute related options
EXECUTE
Turns on all non-extra execute related debug options.
MATCH
Turns on debugging of the main matching loop.
TRIEE
Extra debugging of how tries execute.
INTUIT
Enable debugging of start-point optimisations.
Extra debugging options
EXTRA
Turns on all "extra" debugging options.
BUFFERS
Enable debugging the capture group storage during match.
Warning, this can potentially produce extremely large output.
TRIEM
Enable enhanced TRIE debugging. Enhances both TRIEE and TRIEC.
STATE
Enable debugging of states in the engine.
STACK
Enable debugging of the recursion stack in the engine. Enabling
or disabling this option automatically does the same for
debugging states as well. This output from this can be quite
large.
GPOS
Enable debugging of the \G modifier.
OPTIMISEM
Enable enhanced optimisation debugging and start-point
optimisations. Probably not useful except when debugging the
regexp engine itself.
DUMP_PRE_OPTIMIZE
Enable the dumping of the compiled pattern before the
optimization phase.
WILDCARD
When Perl encounters a wildcard subpattern, (see "Wildcards in
Property Values" in perlunicode), it suspends compilation of the
main pattern, compiles the subpattern, and then matches that
against all legal possibilities to determine the actual code
points the subpattern matches. After that it adds these to the
main pattern, and continues its compilation.
You may very well want to see how your subpattern gets compiled,
but it is likely of less use to you to see how Perl matches that
against all the legal possibilities, as that is under control of
Perl, not you. Therefore, the debugging information of the
compilation portion is as specified by the other options, but
the debugging output of the matching portion is normally
suppressed.
You can use the WILDCARD option to enable the debugging output
of this subpattern matching. Careful! This can lead to
voluminous outputs, and it may not make much sense to you what
and why Perl is doing what it is. But it may be helpful to you
to see why things aren't going the way you expect.
Note that this option alone doesn't cause any debugging
information to be output. What it does is stop the normal
suppression of execution-related debugging information during
the matching portion of the compilation of wildcards. You also
have to specify which execution debugging information you want,
such as by also including the EXECUTE option.
Other useful flags
These are useful shortcuts to save on the typing.
ALL Enable all options at once except BUFFERS, WILDCARD, and
DUMP_PRE_OPTIMIZE. (To get every single option without
exception, use both ALL and EXTRA, or starting in 5.30 on a
"-DDEBUGGING"-enabled perl interpreter, use the -Drv command-
line switches.)
All Enable DUMP and all non-extra execute options. Equivalent to:
use re 'debug';
MORE
More
Enable the options enabled by "All", plus STATE, TRIEC, and
TRIEM.
As of 5.9.5 the directive "use re 'debug'" and its equivalents are
lexically scoped, as are the other directives. However they have both
compile-time and run-time effects.
Exportable Functions
As of perl 5.9.5, the "re" module contains a number of utility functions
that may be optionally exported into the caller's namespace. They are
listed below.
is_regexp($ref)
Returns true if the argument is a compiled regular expression as
returned by "qr//", false if it is not.
This function will not be confused by overloading or blessing. In
internals terms, this extracts the regexp pointer out of the
PERL_MAGIC_qr structure so it cannot be fooled.
regexp_pattern($ref)
If the argument is a compiled regular expression as returned by
"qr//", then this function returns the pattern.
In list context it returns a two element list, the first element
containing the pattern and the second containing the modifiers used
when the pattern was compiled.
my ($pat, $mods) = regexp_pattern($ref);
In scalar context it returns the same as perl would when
stringifying a raw "qr//" with the same pattern inside. If the
argument is not a compiled reference then this routine returns false
but defined in scalar context, and the empty list in list context.
Thus the following
if (regexp_pattern($ref) eq '(?^i:foo)')
will be warning free regardless of what $ref actually is.
Like "is_regexp" this function will not be confused by overloading
or blessing of the object.
regname($name,$all)
Returns the contents of a named buffer of the last successful match.
If $all is true, then returns an array ref containing one entry per
buffer, otherwise returns the first defined buffer.
regnames($all)
Returns a list of all of the named buffers defined in the last
successful match. If $all is true, then it returns all names
defined, if not it returns only names which were involved in the
match.
regnames_count()
Returns the number of distinct names defined in the pattern used for
the last successful match.
Note: this result is always the actual number of distinct named
buffers defined, it may not actually match that which is returned by
regnames() and related routines when those routines have not been
called with the $all parameter set.
regmust($ref)
If the argument is a compiled regular expression as returned by
"qr//", then this function returns what the optimiser considers to
be the longest anchored fixed string and longest floating fixed
string in the pattern.
A fixed string is defined as being a substring that must appear for
the pattern to match. An anchored fixed string is a fixed string
that must appear at a particular offset from the beginning of the
match. A floating fixed string is defined as a fixed string that can
appear at any point in a range of positions relative to the start of
the match. For example,
my $qr = qr/here .* there/x;
my ($anchored, $floating) = regmust($qr);
print "anchored:'$anchored'\nfloating:'$floating'\n";
results in
anchored:'here'
floating:'there'
Because the "here" is before the ".*" in the pattern, its position
can be determined exactly. That's not true, however, for the
"there"; it could appear at any point after where the anchored
string appeared. Perl uses both for its optimisations, preferring
the longer, or, if they are equal, the floating.
NOTE: This may not necessarily be the definitive longest anchored
and floating string. This will be what the optimiser of the Perl
that you are using thinks is the longest. If you believe that the
result is wrong please report it via the perlbug utility.
optimization($ref)
If the argument is a compiled regular expression as returned by
"qr//", then this function returns a hashref of the optimization
information discovered at compile time, so we can write tests around
it. If any other argument is given, returns "undef".
The hash contents are expected to change from time to time as we
develop new ways to optimize - no assumption of stability should be
made, not even between minor versions of perl.
For the current version, the hash will have the following contents:
minlen
An integer, the least number of characters in any string that
can match.
minlenret
An integer, the least number of characters that can be in $&
after a match. (Consider eg " /ns(?=\d)/ ".)
gofs
An integer, the number of characters before pos() to start match
at.
noscan
A boolean, "TRUE" to indicate that any anchored/floating
substrings found should not be used. (CHECKME: apparently this
is set for an anchored pattern with no floating substring, but
never used.)
isall
A boolean, "TRUE" to indicate that the optimizer information is
all that the regular expression contains, and thus one does not
need to enter the regexp runtime engine at all.
anchor SBOL
A boolean, "TRUE" if the pattern is anchored to start of string.
anchor MBOL
A boolean, "TRUE" if the pattern is anchored to any start of
line within the string.
anchor GPOS
A boolean, "TRUE" if the pattern is anchored to the end of the
previous match.
skip
A boolean, "TRUE" if the start class can match only the first of
a run.
implicit
A boolean, "TRUE" if a "/.*/" has been turned implicitly into a
"/^.*/".
anchored/floating
A byte string representing an anchored or floating substring
respectively that any match must contain, or undef if no such
substring was found, or if the substring would require utf8 to
represent.
anchored utf8/floating utf8
A utf8 string representing an anchored or floating substring
respectively that any match must contain, or undef if no such
substring was found, or if the substring contains only 7-bit
ASCII characters.
anchored min offset/floating min offset
An integer, the first offset in characters from a match location
at which we should look for the corresponding substring.
anchored max offset/floating max offset
An integer, the last offset in characters from a match location
at which we should look for the corresponding substring.
Ignored for anchored, so may be 0 or same as min.
anchored end shift/floating end shift
FIXME: not sure what this is, something to do with lookbehind.
regcomp.c says:
When the final pattern is compiled and the data is moved
from the
scan_data_t structure into the regexp structure the
information
about lookbehind is factored in, with the information that
would
have been lost precalculated in the end_shift field for the
associated string.
checking
A constant string, one of "anchored", "floating" or "none" to
indicate which substring (if any) should be checked for first.
stclass
A string representation of a character class ("start class")
that must be the first character of any match.
TODO: explain the representations.
SEE ALSO
"Pragmatic Modules" in perlmodlib.
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