JavaScript regex: why is alternation not ordered?

Question

Given this code:

const regex = /graph|photograph/;
'A photograph'.match(regex);
// Output: [ 'photograph', index: 2, input: 'A photograph', groups: undefined ]

Why is the engine not finding graph first? After looking at similar SO questions and the ECMAScript docs, I can see that

The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative (followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel of the regular expression).

Now, the above quote covers the case /photo|photograph/ where the alternatives share a common beginning, but the case where they share a common ending appears to be governed by a different rule.

I am content with the result I am getting, as in my use case I prefer to get the longest match, not the earliest one, but I would like to know why this happens, so I can be sure this isn't just a coincidence that is bound to change in the future.

The string is parsed from left to right. – Wiktor Stribiżew Apr 28 '20 at 16:50 — Wiktor Stribiżew, Apr 28 '20 at 16:50

Pointy · Accepted Answer · 2022-02-15T15:54:17.953

The alternative graph does not match starting at the third character, but the alternative photograph does. The engine proceeds through the string from left to right.

The ordering you refer to in the question applies when alternatives match from a common starting point in the string. Otherwise, while proceeding through the "haystack" string, the alternatives are all considered. If there's a single match starting from a particular character, then the rest of the regex will proceed with that (and may of course backtrack later).

Whether the engine prefers longer matches from a set of alternatives when there are multiple matches from the same character in the source, I can't say off the top of my head. I would guess it would try the longer one first, to consume more of the string optimistically, because it can always backtrack. However, I don't know that to be actual specified behavior and just thinking about reading the regex semantics in the spec makes my head hurt.

JavaScript regex: why is alternation not ordered?

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