Regular Expressions

Introduction

Examples

Searches with "and" combinations

• TWiki extends the regular expressions with an and search. The delimiter is a semicolon ;. Example search for "form" and "template": form;template

• Use Google if your TWiki site is public. Example search for "form" and "template" at TWiki.org: site:twiki.org +form +template

Advanced

Here is stuff for our UNIX freaks: (copied from 'man egrep')

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.

A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list; if the first character of the list is the caret ^ then it matches any character not in the list. For example, the regular expression [0123456789] matches any single digit.

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive, using the locale's collating sequence and character set. For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales sort characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example.

Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their names are self explanatory, and they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, [[:alnum:]] means [0-9A-Za-z], except the latter form depends upon the C locale and the ASCII character encoding, whereas the former is independent of locale and character set. (Note that the brackets in these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket list.) Most metacharacters lose their special meaning inside lists. To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal - place it last.

The period . matches any single character. The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum]].

The caret ^ and the dollar sign $ are metacharacters that respectively match the empty string at the beginning and end of a line. The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b matches the empty string at the edge of a word, and \B matches the empty string provided it's not at the edge of a word.

A regular expression may be followed by one of several repetition operators:

?	The preceding item is optional and matched at most once.
*	The preceding item will be matched zero or more times.
+	The preceding item will be matched one or more times.
{n}	The preceding item is matched exactly n times.
{n,}	The preceding item is matched n or more times.
{n,m}	The preceding item is matched at least n times, but not more than m times.

Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.

Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either subexpression.

REs are similar to (but more poweful than) the "wildcards" used in the command-line interfaces found in operating systems such as Unix and MS-DOS. REs are used by sophisticated search engines, as well as by many Unix-based languages and tools ( e.g., awk, grep, lex, perl, and sed ).

Examples

Here is stuff for our UNIX freaks:
(copied from 'man grep')

     \c   A backslash (\) followed by any special character is  a
          one-character  regular expression that matches the spe-
          cial character itself.  The special characters are:

               +    `.', `*', `[',  and  `\'  (period,  asterisk,
                    left  square  bracket, and backslash, respec-
                    tively), which  are  always  special,  except
                    when they appear within square brackets ([]).

               +    `^' (caret or circumflex), which  is  special
                    at the beginning of an entire regular expres-
                    sion, or when it immediately follows the left
                    of a pair of square brackets ([]).

               +    $ (currency symbol), which is special at  the
                    end of an entire regular expression.                       

     .    A `.' (period) is a  one-character  regular  expression
          that matches any character except NEWLINE.
 
     [string]
          A non-empty string of  characters  enclosed  in  square
          brackets  is  a  one-character  regular expression that
          matches any one character in that string.  If, however,
          the  first  character of the string is a `^' (a circum-
          flex or caret), the  one-character  regular  expression
          matches  any character except NEWLINE and the remaining
          characters in the string.  The  `^'  has  this  special
          meaning only if it occurs first in the string.  The `-'
          (minus) may be used to indicate a range of  consecutive
          ASCII  characters;  for example, [0-9] is equivalent to
          [0123456789].  The `-' loses this special meaning if it
          occurs  first (after an initial `^', if any) or last in
          the string.  The `]' (right square  bracket)  does  not
          terminate  such a string when it is the first character
          within it (after an initial  `^',  if  any);  that  is,
          []a-f]  matches either `]' (a right square bracket ) or
          one of the letters a through  f  inclusive.   The  four
          characters  `.', `*', `[', and `\' stand for themselves
          within such a string of characters.

     The following rules may be used to construct regular expres-
     sions:

     *    A one-character regular expression followed by `*'  (an
          asterisk)  is a regular expression that matches zero or
          more occurrences of the one-character  regular  expres-
          sion.   If  there  is  any choice, the longest leftmost
          string that permits a match is chosen.

     ^    A circumflex or caret (^) at the beginning of an entire
          regular  expression  constrains that regular expression
          to match an initial segment of a line.

     $    A currency symbol ($) at the end of an  entire  regular
          expression  constrains that regular expression to match
          a final segment of a line.

     *    A  regular  expression  (not  just   a   one-
          character regular expression) followed by `*'
          (an asterisk) is a  regular  expression  that
          matches  zero or more occurrences of the one-
          character regular expression.   If  there  is
          any  choice, the longest leftmost string that
          permits a match is chosen.

     +    A regular expression followed by `+' (a  plus
          sign)  is  a  regular expression that matches
          one or more occurrences of the  one-character
          regular  expression.  If there is any choice,
          the longest leftmost string  that  permits  a
          match is chosen.

     ?    A regular expression followed by `?' (a ques-
          tion  mark)  is  a  regular  expression  that
          matches zero or one occurrences of  the  one-
          character  regular  expression.   If there is
          any choice, the longest leftmost string  that
          permits a match is chosen.

     |    Alternation:    two    regular    expressions
          separated  by  `|'  or NEWLINE match either a
          match for  the  first  or  a  match  for  the
          second.

     ()   A regular expression enclosed in  parentheses
          matches a match for the regular expression.

     The order of precedence of operators at the same parenthesis
     level  is  `[ ]'  (character  classes),  then  `*'  `+'  `?'
     (closures),then  concatenation,  then  `|'  (alternation)and
     NEWLINE.