I'm struggling to write a regex that matches the following requirements:
- up to 20 characters (English letters and numbers)
- may have one optional dash ( - ) but can't start or end with it
I could come up with this patters: ^[a-zA-Z0-9-]{0,20}$ but this one allows for multiple dashes and one may enter the dash at the begin/end of the input string.
CodePudding user response：
You can use
^(?=.{0,20}$)(?:[a-zA-Z0-9] (?:-[a-zA-Z0-9] )?)?$
See the regex demo.
Details:
^- start of string(?=.{0,20}$)- zero to twenty chars allowed in the string(?:- a non-capturing group start:[a-zA-Z0-9]- one or more alphanumeric chars(?:-[a-zA-Z0-9] )?- an optional sequence of a-and one or more alphanumeric chars
)?- end of the non-capturing group, repeat one or zero times (i.e. the pattern match is optional)$- end of string.
CodePudding user response：
Have a try with:
^(?:[^\W_]{1,20}|(?!.{22})[^\W_] -[^\W_] )$
See an online demo
^- Start-line anchor;(?:- Open non-capture group;[^\W_]{1,20}- Match between 1-20 alphanumeric characters;|- Or;(?!.{22})[^\W_] -[^\W_]- Negative lookahead to assert position is not followed by 22 characters, and next we matching 1 alphanumeric characters between an hyphen;
)$- Close non-capture group before matching end-line anchor.
Note that the above assumes upto 20 alphanumeric characters but with one optional hyphen that would take the max count to 21 characters.
CodePudding user response：
You can match two different regex and put both regex in OR:
First regex is to match:
- up to 20 chars (no dash admitted)
^\w{1,20}
Second regex is to match:
- one character at start (no dash)
- optional up to 18 characters or dashes
- one final character (no dash)
^\w[\w-]{1,18}\w$
Complete solution:
^\w{0,20}$|^\w[\w-]{0,18}\w$