Regex to manipulate JavaScript string

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How do I replace all instances of digits within a string pattern with that digit plus an offset.

Say I want to replace all HTML tags with that number plus an offset

strRegEx = /<ol start="(\d+)">/gi;
strContent = strContent.replace(strRegEx, function() {
                                                  /* return $1 + numOffset; */
                                                   });
regex
1 Answers

If you woud return $1 + numOffset; you will get a concatenated string.

If you would turn $1 into an int, you will only return the addition of the group value and the numOffset and will not get the rest of the tag as that is matched, and not returned in the replacement.

You can get the number from capture group 2 and capture what comes before in group 1, and in the function from replace add 3 parameters, the first being the full match (which is not used), and g1, g2 referring to the capture groups.

(<ol start=")(\d+)(?=">)

Then use for example parseInt and add the numOffset.

let strContent = `<ol start="4">`;
const strRegEx = /(<ol start=")(\d+)(?=">)/gi;
const numOffset = 2;
strContent = strContent.replace(strRegEx, function(_, g1, g2) {
  return g1 + (parseInt(g2) + numOffset);
});
console.log(strContent)

Or using a positive lookbehind and a match only without a capture group

(?<=<ol start=")\d+(?=">)


let strContent = `<ol start="4">`;
const strRegEx = /(?<=<ol start=")\d+(?=">)/gi;
const numOffset = 2;
strContent = strContent.replace(strRegEx, function(m) {
  return parseInt(m) + numOffset;
});
console.log(strContent)

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