Matching nested begin/ends

[Abigail-II]

You are all familiar how to match nested balanced parenthesis. But that expressions is easily extendable to larger start and finish markers. Here's how you would match a nested begin/end block.

$re = qr /begin                # Start with 'begin',
          (?:                  # Followed by
             (?>[^be]+)        #    Not a b or e, many times,
            |b(?!egin)         #    Or a b not starting 'begin',
            |e(?!nd)           #    Or an e not starting 'end',
            |(??{ $re })       #    Or a balanced begin/end block
          )*                   # zero or more times
          end/x;               # And an 'end'.