Python regex look-behind strange behaviour with character '^'

Question

In Python's regex: why does regex1 complain about fixed-width look-behinds if regex2 does not? Is ^ (start marker) more than 1 character?

Regex1

(?<=^|b)[0-9]

Regex2

(?<=a|b)[0-9]

Reproduceable in https://regex101.com/r/L5J47R/2

Answer 1

Reason why first regex is nor working in Python because ^ is a zero width match and Python regex engine doesn't support alternation of zero with and non-zero alternations in the lookbehind assertion.

This is however supported in other engines such as Java, PHP, Perl, C# etc.

To solve this problem, you can use this regex:

(?:^|(?<=b))[0-9]

RegEx Demo