NEVER USE (.|\n)!!! - I really mean it.

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Common Regex Issues

Ensuring b doesn't come between a and c
Why is this regex allowing a caret?
How Negative Lookahead Works (more here)
Backtracking with greedy/lazy/possessive quantifiers
Why = or " breaks PHP regexp when using \b? (not only related to PHP)
Why ^ and $ do not work with an alternation like ^a|b$ correctly?

Common Language Specific Regex Issues

JavaScript String#match with global flag does not return captured texts
Unrolling the loop regex technique
Python 2 (and 3) Unicode regex issues: Answer 1, Answer 2 (Japanese!), Answer 3 (Hebrew!)

Regex Tester Related Issues

Regexr.com shows my regex can match 0 characters, and therefore matches infinitely