Javascript regular expression: save matching value

Question

I have the following expression in JS (typescript, but I think everyone understands what it transpiles to):

  markString(text: string) {
    const regEx = new RegExp(this.partToHighlight, 'ig');
    return text.replace(regEx, `<mark>${this.partToHighlight}<\/mark>`);
  }

The problem is that this way, with the 'ig'-option, the matching value can have any case, upper or lower, but is always replaced by the partToHighlight-value. Instead the function should read the matched value, save it and output it surrounded with the HTML-tags. How do I do this? I am pretty sure this is a duplicate question, but I couldn't find the one asked before.

Wiktor Stribiżew · Accepted Answer · 2017-09-27 08:45:37Z

2

You need to replace with the found match, $&:

markString(text: string) {
    const regEx = new RegExp(this.partToHighlight, 'ig');
    return text.replace(regEx, "<mark>$&</mark>");
}

Using $&, you replace with found match with the the same text found and do not need to hardcode the replacement, nor use any callbacks.

See "Specifying a string as a parameter" for more details.

answered Sep 27, 2017 at 8:45

Wiktor Stribiżew

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Comments

cнŝdk · Accepted Answer · 2017-09-27 08:57:57Z

1

As mentionned in comments you will need to use RegExp.lastMatch or $&, to point out to the matched substring, in your replace() method:

const regEx = new RegExp(this.partToHighlight, 'ig');
return text.replace(regEx, `<mark>$&<\/mark>`);

edited Sep 27, 2017 at 8:57

answered Sep 27, 2017 at 8:46

cнŝdk

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2 Comments

Wiktor Stribiżew Over a year ago

Actually, a more appropriate link to the $& is the one I provided in my answer, as you also provided the string replacement backreference based solution (it is the same as mine, just there is no need to escape / in the replacement pattern, as the single \ in the string literal will be removed by the engine anyway).

cнŝdk Over a year ago

@WiktorStribiżew Well pointed out, thank you, I edited my answer.

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