Regex Explainer

In short A regular expression is easier to read when broken into its parts, and this splits one into pieces with a plain-English description of each. The pieces worth knowing by sight are few: character classes in square brackets, quantifiers for how many, groups in parentheses, anchors for start and end, and escapes for characters that would otherwise be special. The two things that trip people up most are greed and anchoring. Quantifiers are greedy by default, so `.*` takes as much as it can and backs off only if the match fails — which is why matching text between two markers with `.*` grabs everything up to the LAST marker rather than the first, and why the lazy form `.*?` exists. Anchoring is the other: without `^` and `$` a pattern matches anywhere in the string, so a validation regex without anchors will happily accept a valid fragment inside invalid input. Flavours differ too. JavaScript, PCRE, Python and POSIX disagree about lookbehind, named groups and shorthand classes, so a pattern that works in one may not compile in another.