Provided by: perl-doc_5.18.2-2ubuntu1.7_all 
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 '/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.
'/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.
"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 /msx on by default, simply
put
use re '/msx';
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'".
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.
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.
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.)
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, prefering
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.