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Match the first group, the second group, or both in a regular expression

I have the following regex:

(electric|acoustic) (guitar|drums)

I need to combine:

  electric guitar electric drums acoustic guitar acoustic drums electric acoustic guitar drums

I tried to use? after both groups, but then it corresponded to everything. Thanks!

Edit:

 <script type="text/javascript"> var s = "electric drums"; if(s.match('^(?:electric()|acoustic())? ?(?:guitar()|drums())?(?:\1|\2|\3|\4)$')){ document.write("match"); } else { document.write("no match"); // returns this } </script>
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2 answers

Use a lookahead based regex like this:

 (?=.*?(?:electric|acoustic|guitar|drums))^(?:electric|acoustic|) ?(?:guitar|drums|)$

Live demo

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One way would be to write:

 ^((electric|acoustic) (guitar|drums)|(electric|acoustic|guitar|drums))$

or (because you don't need parentheses to write)

 ^(?:(?:electric|acoustic) (?:guitar|drums)|(?:electric|acoustic|guitar|drums))$

You can also use the trick if you don't like repeating:

 ^(?:electric()|acoustic())? ?(?:guitar()|drums())?(?:\1|\2|\3|\4)$

(?:\1|\2|\3|\4) ensures that at least one of the previous empty capture groups ( () ) participated in the match.

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