* (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
Recursion
