Javascript Regex to Limit number of characters per line

Question

I'm trying to create a JS Regular Expression to limit the amount of characters per line in an html text area. Ideally I would prefer to have it as a Javascript Regex as it's easier to use for our form validation engine.

I would like to create a regular expression that validates that a string has 10 characters per line, and a max of 3 lines

The idea would be to validate strings like the following ones:

01234567890
a
b

First line here has 11 characters so the whole string should be rejected/

a
b
c
d

This text has 4 lines so it should also be rejected.

I've tried with the following regex but is not working as expected:

^((?:\n)?[^\n]{0,10}){0,3}$

any clues on what I'm doing wrong?

Answer 1

You can use

^.{0,10}(?:\r?\n.{0,10}){0,2}$

See the regex demo

Details

• ^ - start of string
• .{0,10} - zero to ten chars other than line break chars
• (?:\r?\n.{0,10}){0,2} - zero, one or two occurrences of
  • \r?\n - CRLF or LF line ending
  • .{0,10} - zero to ten chars other than line break chars
• $ - end of string.

JavaScript:

const valid_rx = /^.{0,10}(?:\r?\n.{0,10}){0,2}$/;
const testFunc = () => {
    if(!valid_rx.test(document.all.mytextarea.value))
    {
        console.log("Please use correct format.");
        return false;
    }
}

<textarea id="mytextarea" cols="50" rows="10">a
b
c
</textarea>
<button onclick="testFunc()">Click Me</button>