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Regular expression. 20 alphabetic letters and up to 2 of (. - _)

I am trying to create a regex pattern for an html input field that only allows up to 20 combined alphabetic letters and numbers, which can only contain up to two of hyphen (-), underscore (_), and full stop (.)

So something like only two characters are allowed and any number of letters and numbers are allowed, in combination they must be between 4 and 20.

What would be the template for this?

An example (non-functional) version could be like [A-Za-z0-9([\._-]{0,2})]{4,20}

Decision:

I decided to go with @pascalhein @ Honore Doktorr's answer, which is to use lookahead. The final template^(?=[A-Za-z0-9]*([._-][A-Za-z0-9]*){0,2}$)[A-Za-z0-9._-]{4,20}$

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2 answers


You can check the length with lookahead at the beginning:

^(?=.{4,20}$)

Then list all the cases that are allowed for your regex separately:

[A-Za-z0-9]* (no special chars)

[A-Za-z0-9]*[._-][A-Za-z0-9]* (one special char)

[A-Za-z0-9]*[._-][A-Za-z0-9]*[._-][A-Za-z0-9]* (two special chars)

It's not pretty, but I believe it should work. This is the final expression:



^(?=.{4,20}$)([A-Za-z0-9]*|[A-Za-z0-9]*[._-][A-Za-z0-9]*|[A-Za-z0-9]*[._-][A-Za-z0-9]*[._-][A-Za-z0-9]*)$

Edit:

In fact, it would be better to check the number of special characters with a lookahead:

^(?=[A-Za-z0-9]*([._-][A-Za-z0-9]*){0,2}$)[A-Za-z0-9._-]{4,20}$
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I urge you not to use regex if there are functions for the breed you want to achieve.

The advice I give jrs on this topic is to use regex for single viruses, if there is more than one breed, use more regex.



to answer your question:

  • 1 your valid characters from start to finish.

    var 1stregex = / ^ [A-Za-z0-9 ._-] {4,20} $ /;

  • 2 Count must be between 0 and 2, which is .match () in javascript. length.

    var 2ndregex = / ([._-]) /;

    if (myText.match (1stregex) & myText.match (2ndregex) .length <= 2) {isvalid = true; }

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