lang:regexp:recursion

?<S> define the group name S for the expression enclosed in parenthesis. ie (A \g<S>B)?


  * (A \g<S>B)? defines that the expression A \g<S>B should be found 0 or 1 (defined by the ? greedy quantifier)
  * A \g<S>B defines a pattern that:
    * starts with the letter ''A'',
    * followed by the group ''S'' (the recursion)
    * then the letter ''B''




===== Syntax =====

You define a recursive expression by adding the group name
  * In PCRE (perl, php, java, …)
<code>(?R)</code>
  * In Ruby
<code>\g<0></code>

===== Support =====
Recursion is not supported in all regular expression engines. The following engines supports them
  * PCRE (Perl, C, PHP, R…)
  * Ruby (2+)
  * Python with the alternate regex module