Replacing the nth instance of a regex match in Javascript

javascript, regex

Solution

here's something that works:

"23||45||45||56||67".replace(/^((?:[0-9]+\|\|){n})([0-9]+)\|\|/,"$1$2&&")

where n is the one less than the nth pipe, (of course you don't need that first subexpression if n = 0)

And if you'd like a function to do this:

function pipe_replace(str,n) {
   var RE = new RegExp("^((?:[0-9]+\\|\\|){" + (n-1) + "})([0-9]+)\|\|");
   return str.replace(RE,"$1$2&&");
}

Problem

I'm trying to write a regex function that will identify and replace a single instance of a match within a string without affecting the other instances. For example, I have this string: ``` 12||34||56 ``` I want to replace the second set of pipes with ampersands to get this string: ``` 12||34&&56 ``` The regex function needs to be able to handle x amount of pipes and allow me to replace the nth set of pipes, so I could use the same function to make these replacements: ``` 23||45||45||56||67 -> 23&&45||45||56||67 23||34||98||87 -> 23||34||98&&87 ``` I know that I could just split/replace/concat the string at the pipes, and I also know that I can match on `/\|\|/` and iterate through the resulting array, but I'm interested to know if it's possible to write a single expression that can do this. Note that this would be for Javascript, so it's possible to generate a regex at runtime using `eval()`, but it's not possible to use any Perl-specific regex instructions.

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