Answer by tchrist for What is a word boundary in regex?

I talk about what \b-style regex boundaries actually are here.

The short story is that they’re conditional. Their behavior depends on what they’re next to.

# same as using a \b before:(?(?=\w) (?<!\w)  | (?<!\W) )# same as using a \b after:(?(?<=\w) (?!\w)  | (?!\W)  )

Sometimes that isn’t what you want. See my other answer for elaboration.