Using previous expression in other group

Question

/([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?

What language are you using? In PCRE (php)/perl you could use recursion `/([a-z0-9-])@(?1)/i` — HamZa, May 15 '14 at 12:12
I'm using php, was trying it on regex101 that doesn't seem to work. — user3639768, May 15 '14 at 12:14
@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

score 2 · Answer 1 · answered May 15 '14 at 12:06

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If you are using PCRE, then you can do this:

/([a-zA-Z0-9\-])@(?1)/
                ^
              () not needed around the @ sign

answered May 15 '14 at 12:06

Vasili Syrakis

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score 1 · Accepted Answer · edited May 23 '17 at 10:26

1

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 i modifier: /([a-z0-9-])@(?1)/i

Online demo

Further reading:

edited May 23 '17 at 10:26

Community

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answered May 15 '14 at 12:25

HamZa

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Thanks for the great explanation :D – user3639768 May 15 '14 at 12:27
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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If 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

Using previous expression in other group

2 Answers2