How to interpret following regex?

Question

I have a regex but i am not able to interpret it: \w\1.

I thought it would match : aa since it had word a twice and first group would be a word for this regex. But its not behaving in this manner.

Does back referencing work only if we place parentheses around regex ?

Any help would be appreciated. Thanks.

Yes this is a very simple regex in JAVA. I am new to these regex. — Amber
– Amber, Commented Sep 8, 2013 at 18:22
(\w)\1 Captures the matched subexpression and assigns it a zero-based ordinal number. — Rahul Tripathi
– Rahul Tripathi, Commented Sep 8, 2013 at 18:23
@d'alar'cop: Yes and no. The regex itself is okay as it is. But in Java, you construct a regex from a string, and literal strings need extra escapes for backslashes. If you were getting the regexp string from anywhere that is not a literal (from stdin, from DB...), you wouldn't write extra slashies. (That said, the regex doesn't work as intended, because as OP noticed, nothing is captured unless there's braces.) — Amadan
– Amadan, Commented Sep 8, 2013 at 18:25

arshajii · Accepted Answer · 2013-09-08 18:34:18Z

7

\n refers to the nth capturing group. However, there are no capturing groups in your regex to refer to. You likely want:

(\w)\1

As a Java string that would be "(\\w)\\1".

answered Sep 8, 2013 at 18:24

arshajii

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6 Comments

So you mean to say that capturing groups need to be enclosed in "braces"?

@Amber Yes, that's correct. Capturing group 0 always stands for the entire expression.

@Amber They are called parentheses not braces, braces serve a different purpose in regular expressions.

@Amber You are welcome, if this answer solves your problem then please accept it to show credit to the answerer.

@Sniffer I was just waiting for stackoverflow to allow me to accept the answer. (10 min) :)

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Rahul Tripathi · Accepted Answer · 2013-09-08 18:25:08Z

1

(\w)\1 Captures the matched subexpression and assigns it a zero-based ordinal number.

answered Sep 8, 2013 at 18:25

Rahul Tripathi

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