A pattern like this ships in real projects every day:

^(?\[(?:[A-Za-z0-9_.-]+)\])|^(?[A-Za-z_][A-Za-z0-9_]*)\s*=\s*(?"(?:[^"\\]|\\.)*"|'(?:[^'\\]|\\.)*'|[^#\n]*)\s*(?:#.*)?$

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An INI-line parser: a section header or a key–value pair, three quoting modes for the value, trailing comments. Perfectly readable — to the person who wrote it, today. Come back in three months, or hand it to a teammate, and the review comment is always the same: what does this actually match?

Formatting solves this the way it's solved for every other language. The same pattern, laid out by structure:

^ (? \[(?:[A-Za-z0-9_.-]+)\]) | ^ (? [A-Za-z_][A-Za-z0-9_]*) \s* = \s* (? " (?: [^"\\] | \\ . )* " | ' (?: [^'\\] | \\ . )* ' | [^#\n]* ) \s* (?:\#.*)? $

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Two branches instead of one buried | ; three quote styles you can point at; a comment-stripping tail that's obviously a tail.…