/([a-zA-Z0-9\-])(@)([a-zA-Z0-9\-])/
In the regex above group 1 and group 3 contain same expression. Is there another way to use the same expression in another group beside typing it all over again?
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3What language are you using? In PCRE (php)/perl you could use recursion `/([a-z0-9-])@(?1)/i` – HamZa May 15 '14 at 12:12
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I'm using php, was trying it on regex101 that doesn't seem to work. – user3639768 May 15 '14 at 12:14
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1[Oh *really*?](http://regex101.com/r/dG4uV5) – HamZa May 15 '14 at 12:15
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1@user3639768: it's important to put the language tag when you ask something about regex because different languages have different regex features. – Casimir et Hippolyte May 15 '14 at 12:20
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@CasimiretHippolyte thanks I will keep that in my mind :D – user3639768 May 15 '14 at 12:23
2 Answers
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If you are using PCRE, then you can do this:
/([a-zA-Z0-9\-])@(?1)/
^
() not needed around the @ sign
Vasili Syrakis
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If you want to re-use a group, you could use recursion:
/([a-zA-Z0-9\-])(@)(?1)/
(?1) will use the pattern from group 1. Let's now polish your regex:
- Remove unnecessary group:
/([a-zA-Z0-9\-])@(?1)/ - We don't need to escape a hyphen at the end of a character class:
/([a-zA-Z0-9-])@(?1)/ - Let's use the
imodifier:/([a-z0-9-])@(?1)/i
Further reading:
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I'm wondering if the `i` modifier is set, do I need `A-Z` inside `[a-zA-Z0-9\-]` ? – user3639768 May 15 '14 at 12:33
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1If the `i` modifier is set, we don't need `A-Z` since `a-z` is already specified, it will match case insensitive. – HamZa May 15 '14 at 12:34