How to define recursion in Regular expression using recursive subexpression ?

This page is about recursive subexpression in regular expression.

It permits to define string that shows a recursive pattern

In Rugby syntax:

(?<S> (A \g<S> B)?)

where:

• ?<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

You define a recursive expression by adding the group name

• In PCRE (perl, php, java, …)

(?R)

• In Ruby

\g<0>

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