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The second one, I don't even trust it. I think I could match all 3 if they happen in a row.

No, it's fine, it reads like so: Match one of the three choices: "this" or "", "that" or "", or "third_thing" or "". Just like in your first example, the parentheses and alternation operator make sure that it will match only one of the three choices at that place in the regex.

Additionally, there's probably 4 capture groups created as a result.

Correct, but you can use non-capturing (?: ) parens to avoid that, i.e. ((?:this)?|(?:that)?|(?:third_thing)?) would make it have only one capturing group, like your first example. <update> And AnomalousMonk made an excellent point about (?| ) here. </update>

I'd recommend a read of perlrequick, perlretut, and perlre for all of these features and the ones I mentioned earlier. Also, for playing around with regexes and testing out what they do, see my post here.

In reply to Re^3: Regex result being defined when it shouldn't be(?) by haukex
in thread Regex result being defined when it shouldn't be(?) by chenhonkhonk

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