For engines like Hyperscan and RE2's underlying algorithms, alternation works the way you describe. However, for backtracking engines, alternation does not do a

For engines like Hyperscan and RE2's underlying algorithms, alternation works the way you describe. However, for backtracking engines, alternation does not do a search in parallel for all the patterns. Instead, it winds up first looking for the first, then the second, etc., finds the first match, and declares victory. There are particular patterns and inputs where this behavior will be similar to parallel matchers like RE2::Set and Hyperscan, but the general case just won't work.