[anubhavgupta121]

Closes #15782

Problem: The implicit start anchor as a bare '^' was added without any other modifications to the regex logic, so with '| ' (or |, which has lower priority than ^, the ^ only applies to the first alternative, and the others can match anywhere in the string.

Fix: The fix basically wraps the regex with brackets (non-capturing groups, so the anchor '^' applies to every alternative) and then '^' is added so "^(?: ...)" in build_regex(). It's non-capturing, so any further referencing, such as capture group numbering or back-references like \1, isn't affected.

Tests: added cases where a later alternative matches only after offset 0, which the existing tests didn’t cover.