Programming Perl, 4th Edition

Custom Regex Boundaries

A \b for a word boundary and \B for a non-(word boundary) both rely on your current definition of \w (meaning that they change right along with \w if you switch to ASCII semantics with the /a or /aa modifier).

If those aren’t quite the kind of boundaries you’re looking for, you can always write your own boundary assertions based on arbitrary edge conditions, like script boundaries. Here is the definition of \b:

(?(?<=      \w)     # if there is a word character to the left
      (?!   \w)     #    then there must be no word character to the right
  |   (?=   \w)     #    else there must be a  word character to the right
)

And here is the definition of \B:

(?(?<=      \w)     # if there is a word character to the left
      (?=   \w)     #    then there must be a  word character to the right
  |   (?!   \w)     #    else there must be no word character to the right
)

Now that you know exactly how word boundaries and nonboundaries work, you can craft your own boundaries by swapping in your own condition wherever you see \w in the patterns above. You just need to be careful to specify a fixed-width condition so that it can be used in a lookbehind. That means you can’t use things like \X or \R, which are variable width. The easiest way to do that is to use a property or other character class. For example, you could use \p{Greek} for characters in the Greek script—but best add Inherited so you don’t miss the combining characters, so use [\p{Greek}\p{Inherited}] instead.

For example, this might provide regex subroutines suitable for that kind ...