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Man Page for PERLRE

     perlre - Perl regular expressions

     For a description of how to use regular expressions in
     matching operations, see m// and s/// in the perlop manpage.
     The matching operations can have various modifiers, some of
     which relate to the interpretation of the regular expression
     inside.  These are:

         i   Do case-insensitive pattern matching.
         m   Treat string as multiple lines.
         s   Treat string as single line.
         x   Use extended regular expressions.

     These are usually written as "the /x modifier", even though
     the delimiter in question might not actually be a slash.  In
     fact, any of these modifiers may also be embedded within the
     regular expression itself using the new (?...) construct.
     See below.

     The /x modifier itself needs a little more explanation.  It
     tells the regular expression parser to ignore whitespace
     that is not backslashed or within a character class.  You
     can use this to break up your regular expression into
     (slightly) more readable parts.  Together with the
     capability of embedding comments described later, this goes
     a long way towards making Perl 5 a readable language.  See
     the C comment deletion code in the perlop manpage.

     Regular Expressions

     The patterns used in pattern matching are regular
     expressions such as those supplied in the Version 8 regexp
     routines.  (In fact, the routines are derived (distantly)
     from Henry Spencer's freely redistributable reimplementation
     of the V8 routines.) See the section on Version 8 Regular
     Expressions for details.

     In particular the following metacharacters have their
     standard egrep-ish meanings:

         \   Quote the next metacharacter
         ^   Match the beginning of the line
         .   Match any character (except newline)
         $   Match the end of the line
         |   Alternation
         ()  Grouping
         []  Character class

     By default, the "^" character is guaranteed to match only at
     the beginning of the string, the "$" character only at the
     end (or before the newline at the end) and Perl does certain
     optimizations with the assumption that the string contains
     only one line.  Embedded newlines will not be matched by "^"
     or "$".  You may, however, wish to treat a string as a
     multi-line buffer, such that the "^" will match after any
     newline within the string, and "$" will match before any
     newline.  At the cost of a little more overhead, you can do
     this by using the /m modifier on the pattern match operator.
     (Older programs did this by setting $*, but this practice is
     deprecated in Perl 5.)

     To facilitate multi-line substitutions, the "." character
     never matches a newline unless you use the /s modifier,
     which tells Perl to pretend the string is a single line--
     even if it isn't.  The /s modifier also overrides the
     setting of $*, in case you have some (badly behaved) older
     code that sets it in another module.

     The following standard quantifiers are recognized:

         *      Match 0 or more times
         +      Match 1 or more times
         ?      Match 1 or 0 times
         {n}    Match exactly n times
         {n,}   Match at least n times
         {n,m}  Match at least n but not more than m times

     (If a curly bracket occurs in any other context, it is
     treated as a regular character.)  The "*" modifier is
     equivalent to {0,}, the "+" modifier to {1,}, and the "?"
     modifier to {0,1}.  There is no limit to the size of n or m,
     but large numbers will chew up more memory.

     By default, a quantified subpattern is "greedy", that is, it
     will match as many times as possible without causing the
     rest pattern not to match.  The standard quantifiers are all
     "greedy", in that they match as many occurrences as possible
     (given a particular starting location) without causing the
     pattern to fail.  If you want it to match the minimum number
     of times possible, follow the quantifier with a "?" after
     any of them.  Note that the meanings don't change, just the

         *?     Match 0 or more times
         +?     Match 1 or more times
         ??     Match 0 or 1 time
         {n}?   Match exactly n times
         {n,}?  Match at least n times
         {n,m}? Match at least n but not more than m times

     Since patterns are processed as double quoted strings, the
     following also work:

         \t       tab
         \n	  newline 
         \r	  return
	 \f       form feed
         \v	  vertical tab, whatever that is
         \a	  alarm (bell)
	 \e	  escape
	 \033	  octal char
         \x1      hex char
         \l       lowercase next char
         \c[      uppercase next char
         \L       lowercase till \E         
         \U       uppercase till \E
         \E       end case modification
         \Q       quote regexp metacharacters till \E

     In addition, Perl defines the following:

         \w	  Match a "word" character (alphanumeric plus "_")
         \W       Match a non-word character
         \s       Match a whitespace character
         \S       Match a non-whitespace character
         \d       Match a digit character
         \D       Match a non-digit character

     Note that \w matches a single alphanumeric character, not a
     whole word.  To match a word you'd need to say \w+.  You may
     use \w, \W, \s, \S, \d and \D within character classes
     (though not as eieher end of a range).
     Perl defines the following zero-width assertions:

         \b  Match a word boundary
         \B  Match a non-(word boundary)
         \A  Match only at beginning of string
         \Z  Match only at end of string
         \G  Match only where previous m//g left off

     A word boundary (\b) is defined as a spot between two
     characters that has a \w on one side of it and and a \W on
     the other side of it (in either order), counting the
     imaginary characters off the beginning and end of the string
     as matching a \W.  (Within character classes \b represents
     backspace rather than a word boundary.)  The \A and \Z are
     just like "^" and "$" except that they won't match multiple
     times when the /m modifier is used, while "^" and "$" will
     match at every internal line boundary.

     When the bracketing construct ( ... ) is used, \
     matches the digit'th substring.  (Outside of the pattern,
     always use "$" instead of "\" in front of the digit.  The
     scope of $ (and $`, $&, and $') extends to the end of
     the enclosing BLOCK or eval string, or to the next pattern
     match with subexpressions. If you want to use parentheses to
     delimit subpattern (e.g. a set of alternatives) without
     saving it as a subpattern, follow the ( with a ?.  The
     <digit> notation sometimes works outside the current
     pattern, but should not be relied upon.)  You may have as
     many parentheses as you wish.  If you have more than 9
     substrings, the variables $10, $11, ... refer to the
     corresponding substring.  Within the pattern, 10, 11, etc.
     refer back to substrings if there have been at least that
     many left parens before the backreference.  Otherwise (for
     backward compatibilty) 10 is the same as  10, a backspace,
     and 11 the same as  11, a tab.  And so on.  (1 through 9
     are always backreferences.)

     $+ returns whatever the last bracket match matched.  $&
     returns the entire matched string.  ($0 used to return the
     same thing, but not any more.)  $` returns everything before
     the matched string.  $' returns everything after the matched
     string.  Examples:

         s/^([^ ]*) *([^ ]*)/$2 $1/;     # swap first two words

         if (/Time: (..):(..):(..)/) {
             $hours = $1;
             $minutes = $2;
             $seconds = $3;

     You will note that all backslashed metacharacters in Perl
     are alphanumeric, such as \b, \w, \n.  Unlike some other
     regular expression languages, there are no backslashed
     symbols that aren't alphanumeric.  So anything that looks
     like \\, \(, \), \, \{, or \} is always interpreted as
     a literal character, not a metacharacter.  This makes it
     simple to quote a string that you want to use for a pattern
     but that you are afraid might contain metacharacters.
     Simply quote all the non-alphanumeric characters:

      $pattern =~ s/(W)/\$1/g;

     You can also use the built-in quotemeta() function to do
     this.  An even easier way to quote metacharacters right in
     the match operator is to say


     Perl 5 defines a consistent extension syntax for regular
     expressions.  The syntax is a pair of parens with a question
     mark as the first thing within the parens (this was a syntax
     error in Perl 4).  The character after the question mark
     gives the function of the extension.  Several extensions are
     already supported:

     (?#text)  A comment.  The text is ignored.

               This groups things like "()" but doesn't make
               backrefences like "()" does.  So


               is like


               but doesn't spit out extra fields.

               A zero-width positive lookahead assertion.  For
               example, /\w+(?=\t)/ matches a word followed by a
               tab, without including the tab in $&.

               A zero-width negative lookahead assertion.  For
               example /foo(?!bar)/ matches any occurrence of
               "foo" that isn't followed by "bar".  Note however
               that lookahead and lookbehind are NOT the same
               thing.  You cannot use this for lookbehind:
               /(?!foo)bar/ will not find an occurrence of "bar"
               that  is preceded by something which is not "foo".
               That's because the (?!foo) is just saying that the
               next thing cannot be "foo"--and it's not, it's a
               "bar", so "foobar" will match.  You would have to
               do something like /(?foo)...bar/ for that.   We
               say "like" because there's the case of your "bar"
               not having three characters before it.  You could
               cover that this way: /(?:(?!foo)...|^..?)bar/.
               Sometimes it's still easier just to say:

                   if (/foo/ && $` =~ /bar$/)

     (?imsx)   One or more embedded pattern-match modifiers.
               This is particularly useful for patterns that are
               specified in a table somewhere, some of which want
               to be case sensitive, and some of which don't.
               The case insensitive ones merely need to include
               (?i) at the front of the pattern.  For example:

                   $pattern = "foobar";
                   if ( /$pattern/i )

                   # more flexible:

                   $pattern = "(?i)foobar";
                   if ( /$pattern/ )

     The specific choice of question mark for this and the new
     minimal matching construct was because 1) question mark is
     pretty rare in older regular expressions, and 2) whenever
     you see one, you should stop and "question" exactly what is
     going on.  That's psychology...

     Version 8 Regular Expressions

     In case you're not familiar with the "regular" Version 8
     regexp routines, here are the pattern-matching rules not
     described above.

     Any single character matches itself, unless it is a
     metacharacter with a special meaning described here or
     above.  You can cause characters which normally function as
     metacharacters to be interpreted literally by prefixing them
     with a "
     matches a "
     of characters in the target string, so the pattern blurfl
     would match "blurfl" in the target string.

     You can specify a character class, by enclosing a list of
     characters in [], which will match any one of the characters
     in the list.  If the first character after the "[" is "^",
     the class matches any character not in the list.  Within a
     list, the "-" character is used to specify a range, so that
     a-z represents all the characters between "a" and "z",

     Characters may be specified using a metacharacter syntax
     much like that used in C: "\n" matches a newline, "\t" a
     tab, "\r" a carriage return, "\f" a form feed, etc.  More
     generally, \nnn, where nnn is a string of octal digits,
     matches the character whose ASCII value is nnn.  Similarly,
     \xnn, where nn are hexidecimal digits, matches the character
     whose ASCII value is nn. The expression \cx matches the
     ASCII character control-x.  Finally, the "." metacharacter
     matches any character except "\n" (unless you use /s).

     You can specify a series of alternatives for a pattern using
     "|" to separate them, so that fee|fie|foe will match any of
     "fee", "fie", or "foe" in the target string (as would
     f(e|i|o)e).  Note that the first alternative includes
     everything from the last pattern delimiter ("(", "[", or the
     beginning of the pattern) up to the first "|", and the last
     alternative contains everything from the last "|" to the
     next pattern delimiter.  For this reason, it's common
     practice to include alternatives in parentheses, to minimize
     confusion about where they start and end.  Note also that
     the pattern (fee|fie|foe) differs from the pattern
     [fee|fie|foe] in that the former matches "fee", "fie", or
     "foe" in the target string, while the latter matches
     anything matched by the classes [fee], [fie], or [foe] (i.e.
     the class [feio]).

     Within a pattern, you may designate subpatterns for later
     reference by enclosing them in parentheses, and you may
     refer back to the nth subpattern later in the pattern using
     the metacharacter \n. Subpatterns are numbered based on the
     left to right order of their opening parenthesis.  Note that
     a backreference matches whatever actually matched the
     subpattern in the string being examined, not the rules for
     that subpattern.  Therefore, ([0|0x])\d*\s\1\d* will match
     "0x1234 0x4321",but not "0x1234 01234", since subpattern 1
     actually matched "0x", even though the rule [0|0x] could
     potentially match the leading 0 in the second number.


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