How do I make a regex with a lookbehind assertion that still works at the start of a string

Question

I need to emulate the behavior of \b at the start of a string, where I'm adding additional characters to the set that count as a word boundary. Right now I'm using something like:

"(?<=\\W|\\p{InCJKUnifiedIdeographs})foo"

This works as I would like, unless I'm at the start of the string being matched: in which case the assertion fails and I don't get a hit. What I want is the equivalent of match if I'm at the start of the string or foo is preceded by a non-word character or an ideograph. But I can't get the right incantation to support that.

Any thoughts? Or is this impossible?

Thanks in advance.

Extant

What do you mean by match if i am at the start of the string? That would capture all strings because all strings have a 'start of string' – Martita

It doesn't: if I use the aforementioned regex against the string "foo foobar baz" it will not find 'foo' because the look behind fails. – Extant

In most cases, you can get what you want by reversing the condition: (?<![\w\P{InCJKUnifiedIdeographs}]). I'd add it as an answer, but I don't have time to test it. – Katharina

Answer 1

"(?<=^|\\W|\\p{InCJKUnifiedIdeographs})foo"

Just add the start-of-string anchor to the lookbehind conditions.

Fisticuffs

Thanks Robert, that works like a charm. Somehow in the various combinations I experimented with I didn't try the most obvious. – Extant

Adding a carot leads to error in my case ((?<=^| )is(?= |$) regex101.com/r/vD5iH9/21 – Shute

@СашкоЛихенко That's a limitation of Python's regex engine. It allows only "fixed width" look-behinds, and the length of ^ (zero/null?/NaN?) is obviously different than ` ` (one). – Fisticuffs

@СашкоЛихенко See if using the word-boundary match will work for you, e.g. \bis\b or something like that. – Fisticuffs

Thanks, this was not very easy to find in documents... I tried variants like (?<=\\^|\\W... or (?<=(\\^|\\W... etc. – Angy