You should not be permitted to write production code if you do not have an journeyman licence in regular expressions or floating point math. —Rob Pike
a regular expression is two things wearing one syntax: a seventy-year-old theorem about finite automata, and the single most-used text-processing tool on unix. 𐃏 this page takes both seriously: the theory tells you exactly what the notation can and cannot express, and the theory’s failure modes (backtracking blowups, backreference NP-hardness, the html-parsing folklore) are precisely where practitioners get burned.