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Section: Perl Programmers Reference Guide (3pm)
Updated: 2018-03-23


re - Perl pragma to alter regular expression behaviour 


    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.) 



'taint' mode

When "use re 'taint'" is in effect, and a tainted string is the targetof a regexp, the regexp memories (or values returned by the m// operatorin list context) are tainted. This feature is useful when regexp operationson tainted data aren't meant to extract safe substrings, but to performother transformations. 

'eval' mode

When "use re 'eval'" is in effect, a regexp is allowed to contain"(?{ ... })" zero-width assertions and "(??{ ... })" postponedsubexpressions that are derived from variable interpolation, rather thanappearing literally within the regexp. That is normally disallowed, sinceit is apotential security risk. Note that this pragma is ignored when the regularexpression is obtained from tainted data, i.e. evaluation is alwaysdisallowed with tainted regular expressions. See ``(?{ code })'' in perlre and ``(??{ code })'' in perlre.

For the purpose of this pragma, interpolation of precompiled regularexpressions (i.e., the result of "qr//") is not considered variableinterpolation. Thus:


is allowed if $pat is a precompiled regular expression, evenif $pat contains "(?{ ... })" assertions or "(??{ ... })" subexpressions. 

'strict' mode

Note that this is an experimental feature which may be changed or removed in afuture Perl release.

When "use re 'strict'" is in effect, stricter checks are applied thanotherwise when compiling regular expressions patterns. These may cause morewarnings to be raised than otherwise, and more things to be fatal instead ofjust warnings. The purpose of this is to find and report at compile time somethings, which may be legal, but have a reasonable possibility of not being theprogrammer'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 nototherwise, is the pattern


The "\x" construct without curly braces should be followed by exactly twohex digits; this one is followed by three. This currently evaluates asequivalent to


that is, the character whose code point value is 0xAB, followed by theletter "C". But since "C" is a a hex digit, there is a reasonable chancethat the intent was


that is the single character at 0xABC. Under 'strict' it is an error tonot follow "\x" with exactly two hex digits. When not under 'strict' awarning is generated if there is only one hex digit, and no warning is raisedif there are more than two.

It is expected that what exactly 'strict' does will evolve over time as wegain experience with it. This means that programs that compile under it intoday's Perl may not compile, or may have more or fewer warnings, in futurePerls. There is no backwards compatibility promises with regards to it. Alsothere are already proposals for an alternate syntax for enabling it. Forthese 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 byinterpolating into another pattern, outside of 'strict', it is not checkedagain for strictness. This is because if it works under strict it must workunder non-strict. 

'/flags' mode

When "use re '/flags'" is specified, the given flags are automaticallyadded 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 thegiven flags.

For example, if you want all your regular expressions to have /msxx on bydefault, 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'".


    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 regularexpressions. Turning off one of these flags when it is active reverts tothe behaviour specified by whatever other pragmata are in scope. Forexample:

    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 whencompiling and using regular expressions. The output is the same as thatobtained by running a "-DDEBUGGING"-enabled perl interpreter with the-Dr switch. It may be quite voluminous depending on the complexityof the match. Using "debugcolor" instead of "debug" enables aform of output that can be used to get a colorful display on terminalsthat understand termcap color sequences. Set $ENV{PERL_RE_TC} to acomma-separated list of "termcap" properties to use for highlightingstrings 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 arelexically scoped, as the other directives are. However they have bothcompile-time and run-time effects.

See ``Pragmatic Modules'' in perlmodlib. 

'Debug' mode

Similarly "use re 'Debug'" produces debugging output, the differencebeing that it allows the fine tuning of what debugging output will beemitted. Options are divided into three groups, those related tocompilation, those related to execution and those related to specialpurposes. The options are as follows:
Compile related options
Turns on all compile related debug options.
Turns on debug output related to the process of parsing the pattern.
Enables output related to the optimisation phase of compilation.
Detailed info about trie compilation.
Dump the final program out after it is compiled and optimised.
Dump the flags associated with the program
Print output intended for testing the internals of the compile process
Execute related options
Turns on all execute related debug options.
Turns on debugging of the main matching loop.
Extra debugging of how tries execute.
Enable debugging of start-point optimisations.
Extra debugging options
Turns on all ``extra'' debugging options.
Enable debugging the capture group storage during match. Warning,this can potentially produce extremely large output.
Enable enhanced TRIE debugging. Enhances both TRIEEand TRIEC.
Enable debugging of states in the engine.
Enable debugging of the recursion stack in the engine. Enablingor disabling this option automatically does the same for debuggingstates as well. This output from this can be quite large.
Enable debugging of the \G modifier.
Enable enhanced optimisation debugging and start-point optimisations.Probably not useful except when debugging the regexp engine itself.
Dump offset information. This can be used to see how regops correlateto the pattern. Output format is


Where 1 is the position of the first char in the string. Note that positioncan be 0, or larger than the actual length of the pattern, likewise lengthcan be zero.

Enable debugging of offsets information. This emits copiousamounts of trace information and doesn't mesh well with otherdebug options.

Almost definitely only useful to people hackingon the offsets part of the debug engine.

Other useful flags
These are useful shortcuts to save on the typing.
Enable all options at once except OFFSETS, OFFSETSDBG and BUFFERS.(To get every single option without exception, use both ALL and EXTRA.)
Enable DUMP and all execute options. Equivalent to:

  use re 'debug';
Enable the options enabled by ``All'', plus STATE, TRIEC, and TRIEM.

As of 5.9.5 the directive "use re 'debug'" and its equivalents arelexically scoped, as are the other directives. However they have bothcompile-time and run-time effects. 

Exportable Functions

As of perl 5.9.5 're' debug contains a number of utility functions thatmay be optionally exported into the caller's namespace. They are listedbelow.
Returns true if the argument is a compiled regular expression as returnedby "qr//", false if it is not.

This function will not be confused by overloading or blessing. Ininternals terms, this extracts the regexp pointer out of thePERL_MAGIC_qr structure so it cannot be fooled.

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 elementcontaining the pattern and the second containing the modifiers used whenthe 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 compiledreference 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 overloadingor blessing of the object.

If the argument is a compiled regular expression as returned by "qr//",then this function returns what the optimiser considers to be the longestanchored fixed string and longest floating fixed string in the pattern.

A fixed string is defined as being a substring that must appear for thepattern to match. An anchored fixed string is a fixed string that mustappear at a particular offset from the beginning of the match. A floatingfixed string is defined as a fixed string that can appear at any point ina 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 positioncan 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 areequal, the floating.

NOTE: This may not necessarily be the definitive longest anchored andfloating string. This will be what the optimiser of the Perl that youare using thinks is the longest. If you believe that the result is wrongplease report it via the perlbug utility.

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.
Returns a list of all of the named buffers defined in the last successfulmatch. If $all is true, then it returns all names defined, if not it returnsonly names which were involved in the match.
Returns the number of distinct names defined in the pattern usedfor the last successful match.

Note: this result is always the actual number of distinctnamed buffers defined, it may not actually match that which isreturned by "regnames()" and related routines when those routineshave not been called with the $all parameter set.



``Pragmatic Modules'' in perlmodlib.



'taint' mode
'eval' mode
'strict' mode
'/flags' mode
'debug' mode
'Debug' mode
Exportable Functions

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