How get only a specific part of a string using a regular expression?

Question

In an input field I have the value A=123 and I need the JavaScript code to get only the 123 part.

This is what I tried so far:

function formAnswer() {
    var input = document.getElementById('given').value;
    var match = input.match(/A=.*/);
    document.getElementById('result').value = match;
}

<input type="text" id="given" placeholder="Given" value="A=123"/>
<input type="text" id="result" placeholder="Result"/>
<input type="button" value="Click!" onClick="formAnswer();"/>

But this still gets the value A=123. What am I doing wrong here?

Just an other note: you have no textarea, you only have text inputs. — Kévin Bibollet
– Kévin Bibollet, Commented May 13, 2019 at 18:03
If there can be digits only and that is the only allowed text you could use anchors to assert the start and the end of the string and capture in a group 1+ digits ^A=(\d+)$ See regex101.com/r/6FuzJR/1 — The fourth bird
– The fourth bird, Commented May 13, 2019 at 20:10

Kévin Bibollet · Accepted Answer · 2019-05-13 18:01:48Z

4

Use parenthesis in your regular expression to catch what's after A=. The caught value will be available with match[1] (match[0] will be the entire expression).

function formAnswer() {
  let input = document.getElementById('given').value;
    match = input.match(/A=(.*)/);
    
  document.getElementById('result').value = match[1];
}

<input type="text" id="given" placeholder="Given"/>
<input type="text" id="result" placeholder="Result"/>
<input type="button" onClick="formAnswer();"/>

answered May 13, 2019 at 18:01

Kévin Bibollet

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Comments

vrintle · Accepted Answer · 2019-05-13 18:24:40Z

1

You can try this regex: (?<=A=).*

It simply searches for a string which is preceded by "A="

function formAnswer() {
  var input = document.getElementById('given').value;
  var match = input.match(/(?<=A=).*/);
  document.getElementById('result').value = match;
}

<input type="text" id="given" placeholder="Given" value="A=123" />
<input type="text" id="result" placeholder="Result" />
<input type="button" onClick="formAnswer();" value="Check" />

edited May 13, 2019 at 18:24

answered May 13, 2019 at 18:03

vrintle

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3 Comments

vrintle Over a year ago

Hi @JennyO'Reilly, it seems my eyes aren't working. It would be helpful if you will give some more info. (like, error message, input, etc)

Code Maniac Over a year ago

Error is due to compatibility of browser as lookbacks are not implemented in all the browsers

Krisztián Balla Over a year ago

Sorry. I deleted the original comment. The error I get in Firefox 66.0.5 (Win64) is this one: SyntaxError: invalid regexp group. As Code Maniac pointed out, regex lookbacks in JavaScript are not supported by most browsers, as you can see here: kangax.github.io/compat-table/es2016plus/…

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