CODEWITHSUNDEEP
rubyregex
freenight
freenight

Reputation: 1089

regular expression back referencing

why this snippet:

'He said "Hello"' =~ /(\w)\1/

matches "ll"? I thought that the \w part matches "H", and hence \1 refers to "H", thus nothing should be matched? but why this result?

Upvotes: 1

Views: 144

Answers (4)

Comradin
Comradin

Reputation: 202

The point is, "\1" means one repetition of the "(\w)" block, only the letter "l" is doubled and will match your regex.

A nice page for toying around with ruby and regular expressions is Rubular

Upvotes: 0

Mark Byers
Mark Byers

Reputation: 838026

You're thinking of /^(\w)\1/. The caret symbol specifies that the match must start at the beginning of the line. Without that, the match can start anywhere in the string (it will find the first match).

Upvotes: 3

Konrad Rudolph
Konrad Rudolph

Reputation: 545518

I thought that the \w part matches "H"

\w matches any alphanumerical character (and underscore). It also happens to match H but that’s not terribly interesting since the regular expression then goes on to say that this has to be matched twice – which H can’t in your text (since it doesn’t appear twice consecutively), and neither is any of the other characters, just l. So the regular expression matches ll.

Upvotes: 3

SilentGhost
SilentGhost

Reputation: 319541

and you're right, nothing was matched at that position. then regex went further and found match, which it returned to you.

\w is of course matches any word character, not just 'H'.

Upvotes: 2

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