Reputation: 1303

Regex on limiting one occurrence of particular character

I am working on a regex that allow a character 'x' and any digit from 0-9.

below are the rules.

^(x|[0-9])(x|[0-9])(x|[0-9])(x|[0-9])$

My current regex only able rule 1 and 2, but it doesn't filter out those with more than one 'x'

x000 //ok
xxxx //ok , but should be not ok
23xx //ok , but should be not ok
a90d //not ok
11x1 //ok
x213 //ok

Since the regex will be used for validation in keyup so the rule must concern when the user type from one to four keyup.

Updated rules

Upvotes: 4

Answers (3)

Reputation: 425378

Use a look ahead for the “only 1 x” condition:

^(?=\d*x\d*$).{4}$

Upvotes: 2

Reputation: 627498

You may use

/^(?=[0-9x]{4}$)[0-9]*x[0-9]*$/

/^(?=[\dx]{4}$)\d*x\d*$/

Details

^ - start of string
(?=[\dx]{4}$) - a positive lookahead checking that there are exactly 4 digits or x from the start up to the end of string
\d* - 0+ digits
x - an x
\d* - 0+ digits
$ - end of string.

Note that in this case, you may even reduce the whole pattern to

/^(?=.{4}$)\d*x\d*$/
  ^^^^^^^^^

to just check the length of the string without checking the type of chars (since digits and x are non-linebreak chars).

Upvotes: 3

Reputation: 522741

One option uses lookahead to handle the one x requirement:

^(?=.*x)(?!.*x.*x)[0-9x]{4}$

See the regex demo.

Upvotes: 0