Ruby string does not match expression

Question

I have this ruby expression as below

  (a|bc)(d?|e)*

when i use rubular to test out possible strings that fit this expression, I have some strings that I dont understand why they dont fit

the strings are "ade", it matches "ad" but does not match the "e". Anyone can help?

Behram Mistree · Accepted Answer · 2012-09-08 06:04:17Z

1

The second part of the regular expression you entered (d?|e)* is the problem. Putting the ? on the d says, match d 0 or 1 times. When you run through the string ade, the regex matches a, then d, then d 0 times... If you instead changed it to (a|bc)(d|e)*, it would match ade, and seem to have the semantics that you're looking for.

answered Sep 8, 2012 at 6:04

Behram Mistree

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kkh Over a year ago

just wondering, so any part of the string that contains an e will not match the regular expression?

SwiftMango · Accepted Answer · 2012-09-08 06:17:06Z

1

(d?)* is a non-greedy match and e* will be "short circuited" by logic or. It will match as few as possible.

I don't know why you put a question mark there. Just use

(a|bc)(d|e)*

Will be fine.

answered Sep 8, 2012 at 6:17

SwiftMango

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