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From: Thomas 'PointedEars' Lahn <PointedEars@web.de>
Newsgroups: comp.lang.javascript
Subject: Re: Help with regexp
Date: Sat, 07 Aug 2021 21:56:28 +0200
Organization: PointedEars Software (PES)
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Andrew Poulos wrote:

> I have a string that looks like this
> 
> let str = 'linear-gradient(90deg, #ff0000 50%, #00ff00 100%)"
> 
> I need to convert the hex values to rgba so I've written a function
> 
> const convertHex2Rgba = (hex, alpha = 1) => {
>    ...
>    return `rgba(${r},${g},${b},${alpha})`;
> };
> 
> I'm stuck trying to write a regexp that replaces the hex values (that
> may have either 3 or 6 characters - plus the #) with the value returned
> from convertHex2Rgba.
> 
> This (one of my many failed attempts):
> 
> str.replace(/#[0-9a-fA-F]/, convertHex2Rgba('$1'));
> 
> does nothing.

Of course not.

  1. You are not using the return value of the “replace” method.

  2. You are not passing your function a proper value for the formal
     parameter “hex”, but the string value '$1' instead.
     ('$1' is only expanded with the first captured subpattern if
      the entire argument is a string value.)

  3. You are not matching that which you intend to match.
     (You are only matching a single hexadecimal digit.)

This should work:

  const generateHex2Rgba =
    ((alpha = 1) => ((hex, red, green, blue, count) => {
      let [r, g, b] = [red, green, blue].map(s => parseInt(s, 16));
      return `rgba(${r},${g},${b},${alpha})`;
    }));

  str = str.replace(
    /#([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})/gi,
    generateHex2Rgba()
  );

Why you want to do that remains a mystery, though.

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