04 — Regex

Ruby's regex engine is Onigmo. Its syntax is Perl's, with differences in exactly the places that are easy to miss when a pattern moves between languages — anchors, the meaning of /m, the letter \h — and with an engine that has, since Ruby 3.2, defended itself against the most famous way to make a backtracking regex take forever.

Lesson	The one thing
^ and $ match at every line	\A and \z for validation, and a three-language table
\w is ASCII, [[:alpha:]] is not, and \b sides with Unicode	why /\b\w+\b/ cannot find café
\h is a hex digit in Ruby and a blank in Perl	one letter, three meanings
Named captures become local variables — sometimes	the parser rule behind year appearing from nowhere
Backtracking has a cache, and a timeout	Regexp.linear_time? and Regexp.timeout