Regular expression of two digit number where two digits are not same

Question

I am trying to write a regular expression that will match a two digit number where the two digits are not same.

I have used the following expression:

^([0-9])(?!\1)$

However, both the strings "11" and "12" are not matching. I thought "12" would match. Can anyone please tell me where I am going wrong?

`#` is missing in the input - use `$` to denote the end of string and allow 1 more digit. — Wiktor Stribiżew, Dec 11 '15 at 08:47

score 5 · Accepted Answer · answered Dec 11 '15 at 08:49

You need to allow matching 2 digits. Your regex ^([0-9])(?!\1)$ only allows 1 digit string. Note that a lookahead does not consume characters, it only checks for presence or absence of something after the current position.

Use

^(\d)(?!\1)\d$
           ^^

See demo

Explanation of the pattern:

^ - start of string
(\d) - match and capture into Group #1 a digit
(?!\1) - make sure the next character is not the same digit as in Group 1
\d - one digit
$ - end of string.

Regular expression of two digit number where two digits are not same

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