Regex pattern for string having spaces only at end

Question

I have a requirement where I need to match string which satisfies all of the below requirements -

String must be of length 12
String can have only following characters - Alphabets, Digits and Spaces
Spaces if any must be at the end of the string. Spaces in between are not allowed.

I have tried with below regex -

"^[0-9a-zA-Z\s]{12}$"

Above regex is satisfying requirement #1 and #2 but not able to satisfy #3. Please help me to achieve the requirements. Thanks in advance !!

Wiktor Stribiżew · Accepted Answer · 2021-12-01T15:37:40.053

2

You can use

^(?=.{12}$)[0-9a-zA-Z]*\s*$

If at least one letter must exist:

^(?=.{12}$)[0-9a-zA-Z]+\s*$

Details:

^ - start of string
(?=.{12}$) - the string must contain 12 chars
[0-9a-zA-Z]* - zero or more alphanumeroics
\s* - zero or more whitespaces
$ - end of string.

See the regex demo.

edited Dec 01 '21 at 15:37

answered Dec 01 '21 at 15:34

Wiktor Stribiżew

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This is working to match, what about a solution that enables searching? – mozway Dec 01 '21 at 15:35
@mozway This comment is irrelevant to the current requirements. The OP question is about validation and not extraction from longer text. – Wiktor Stribiżew Dec 01 '21 at 15:36
Thus my question, I am curious if you had an answer to that ;) – mozway Dec 01 '21 at 15:37
@mozway Then please go ahead and post it. I do not know your exact requirements. – Wiktor Stribiżew Dec 01 '21 at 15:38
Thanks..it solved my problem. But can you please explain me (?=.{12}$) how this is working ? – Suraj Pandey Dec 01 '21 at 18:12
@SurajPandey See [Restricting Character length in Regular expression](https://stackoverflow.com/a/32477224/3832970). It is a positive lookahead, it just checks if its pattern can eb matched immediately on the right. If it matches, matching can proceed, else, the match is failed. – Wiktor Stribiżew Dec 01 '21 at 19:10

Casimir et Hippolyte · Answer 2 · 2021-12-01T15:54:53.060

Use a non-word boundary \B:

^(?:[a-zA-Z0-9]|\s\B){12}$

demo

With it, a space can't be followed by a letter or a digit, but only by a non-word character (a space here) or the end of the string.

To ensure at least one character that isn't blank:

^[a-zA-Z0-9](?:[a-zA-Z0-9]|\s\B){11}$

Note that with PCRE you have to use the D (DOLLAR END ONLY) modifier to be sure that $ matches the end of the string and not before the last newline sequence. Or better replace $ with \z. There isn't this kind of problem with Python and the re module.

score 0 · Answer 3 · answered Dec 01 '21 at 15:34

You may use this regex:

^(?!.*\h\S)[\da-zA-Z\h]{12}$

RegEx Demo

RegEx Details:

^: Start
(?!.*\h\S): Negative lookahead to fail the match if a whitespace is followed by a non-whitespace character
[\da-zA-Z\h]{12}: Match 12 characters of alphanumerics or white space
$: End

Regex pattern for string having spaces only at end

3 Answers3