Indicate what flavor of regex we're using about circleci-docs

[felicianotech]

Yes I like this. We can do a guide because there's a blog post similar to this I plan on doing anything. Then I can link back. Great idea Eric.

[gladiatr72]

TL;DR

PCRE/POSIX hybrid

etc...

This ticket contains no real answer, yet it seems to have fairly high ranking for searches on this issue:

The solitary mention of the regex engine in use is this line in the workflows doc.

CircleCI branch and tag filters support the Java variant of regex pattern matching

... which is similar to that found in libpcre (subtle syntactic differences)

[smart-alek]

Closing in favor of 2.0 docs, but may still be a good idea to address regex for 2.0 if it becomes an issue.

[GaxZE]

I am having this issue with case sensitive and branches. I hope to have a build run on a specific regex which ignores case sensitivity. I have so far been unsuccessful. Any tips here would be excellent.
/some-branch/i
/(?i)some-branch/

Edit, the reason it wasn't working for me was because my regex was as follows:

/(?i)^some-branch/ when it needed to be /^(?i)some-branch/