CODEWITHSUNDEEP
regex
daremkd
daremkd

Reputation: 8424

Regex confusion - ? inside and outside of parenthesis

This regex:

(a)?b\1c

does not match "bc" while this one:

(a?)b\1c

does match it. Why is this? I thought these statements are identical.

Upvotes: 6

Views: 177

Answers (3)

zessx
zessx

Reputation: 68790

In your first example (a)?b\1c, \1 refers to your (a) group, it means you must have an a :

enter image description here

  • abac will match
  • bac will match
  • bc won't match

In your second example (a?)b\1c, \1 refers to (a?), where a is optional :

enter image description here

  • abac will match
  • bac won't match
  • bc will match

The back reference doesn't care of your external ? (in the first example), it only takes care of what is inside parenthesis.

Upvotes: 6

Ibrahim Najjar
Ibrahim Najjar

Reputation: 19423

It's a bit confusing, but let's see, I will start with the second regular expression:

(a?)b\1c

When this tries to match bc it first tries (a?) but since there is no a in bc, () will capture the empty string "" so when we later refer to it in the string using \1, \1 will match the empty string which is always possible.

Now let's go to the second case:

(a)?b\1c

(a) will try to match a but fails, but since the entire group (a)? is optional, the regular expression continues, now it tries to find a b OK, then \1 but (a)? didn't match anything, even the empty string so the match fails.

So the difference between the two regex is that in (a?) the capturing group captures an empty string which can be referenced later and matched successfully using \1, but (a)? creates an optional capturing group that didn't match anything so referencing it later using \1 will always fails unless the group actually matched an a.

Upvotes: 3

Idriss Neumann
Idriss Neumann

Reputation: 3838

In the firs version, parentheses catch a so \1 returns a.

In the second regex, parentheses catch a? so \1 returns a? which means "0 or 1 a".

As a is optional in the second regex, bc match so well the end of the second regex (b\1c)

Upvotes: 2

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