re
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 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
'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.
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. The options are as follows:
Compile related options
COMPILE
Turns on all 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 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.
OFFSETS
Dump offset information. This can be used to see how regops
correlate to the pattern. Output format is
NODENUM:POSITION[LENGTH]
Where 1 is the position of the first char in the string. Note
that position can be 0, or larger than the actual length of the
pattern, likewise length can be zero.
OFFSETSDBG
Enable debugging of offsets information. This emits copious
amounts of trace information and doesn't mesh well with other
debug options.
Almost definitely only useful to people hacking on the offsets
part of the debug engine.
Other useful flags
These are useful shortcuts to save on the typing.
ALL Enable all options at once except OFFSETS, OFFSETSDBG and
BUFFERS. (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 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 're' debug 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 $ref actually is.
Like "is_regexp" this function will not be confused by overloading
or blessing of the object.
regmust($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.
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.
SEE ALSO
"Pragmatic Modules" in perlmodlib.
perl v5.30.0 2023-11-23 re(3perl)
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