Building a Javascript Regex from string

Question

I'm trying to return the number of times a letter appears in a word.

I'm passing the letter to a function like so

function getCount(letter)
{
    var pattern = '/' + letter + '/g';
    var matches = word.match(pattern);
    return matches.length;
}

Unfortunately matches is null so I'm unable to call length on it, I know the letter appears in the word as I've already checked that

word.indexOf(letter) > -1

I suspect the problem is with the way I'm building or evaluating pattern

Denys Séguret · Accepted Answer · 2013-02-25T10:11:29.017

8

Here's how you build a non literal regular expression :

var pattern = new RegExp(letter, 'g');

See the MDN on building a regular expression.

And here's a simpler solution to count the occurrences of the letter :

return word.split(letter).length-1;

edited Feb 25 '13 at 10:11

answered Feb 22 '13 at 18:19

Denys Séguret

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If I could give +2 for the alternative solution, I would. Sadly +1 is all I can offer. – Niet the Dark Absol Feb 22 '13 at 18:22
@Kolink Unfortunately I just realized my alternate solution wasn't so good. It's less elegant now... – Denys Séguret Feb 22 '13 at 18:23
Or `word.split(letter).length-1`. – MikeM Feb 22 '13 at 18:33
@MikeM This was my first answer but the problem is that it doesn't work if the letter is at start or end of the word. – Denys Séguret Feb 22 '13 at 18:37
2

It does for me! `'ergo'.split('e').length - 1; // 1`. It's a neat solution anyhow. – MikeM Feb 22 '13 at 18:39
I've used the `var pattern = new RegExp(letter, 'g');` solution. – Luke Feb 22 '13 at 18:42

score 2 · Answer 2 · edited May 23 '17 at 12:04

2

You can do this:

function hasClass(letter) {
    var pattern = new RegExp(letter,'g'); // Create a regular expression from the string
    var matches = word.match(pattern);
    return matches;

Ref: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/RegExp

This was also answered here: javascript new regexp from string

edited May 23 '17 at 12:04

Community

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answered Feb 22 '13 at 18:28

Jeff Shaver

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Do you realize that this is the answer I made 10 minutes before ? – Denys Séguret Feb 22 '13 at 18:31
1

I had typed it all and was just grabbing the reference when the message came up, so I just clicked submit anyway. – Jeff Shaver Feb 22 '13 at 18:36
sorry for not catching the dupe. – Luke Feb 22 '13 at 18:41

Building a Javascript Regex from string

2 Answers2