Replace all character matches that are not escaped with backslash

Question

I am using regex to replace ( in other regexes (or regexs?) with (?: to turn them into non-matching groups. My expression assumes that no (?X structures are used and looks like this:

(
  [^\\]     - Not backslash character
  |^        - Or string beginning
)
(?:
  [\(]      - a bracket
)

Unfortunatelly this doesn't work in case that there are two matches next to each other, like in this case: how((\s+can|\s+do)(\s+i)?)?

With lookbehinds, the solution is easy:

/(?<=[^\\]|^)[\(]/g

But javascript doesn't support lookbehinds, so what can I do? My searches didn't bring any easy universal lookbehind alternative.

Answer 1

Use lookbehind through reversal:

function revStr(str) {
    return str.split('').reverse().join('');
}

var rx = /[(](?=[^\\]|$)/g;
var subst = ":?(";

var data = "how((\\s+can|\\s+do)(\\s+i)?)?";
var res = revStr(revStr(data).replace(rx, subst)); 
document.getElementById("res").value = res;

<input id="res" />

Note that the regex pattern is also reversed so that we could use a look-ahead instead of a look-behind, and the substitution string is reversed, too. It becomes too tricky with longer regexps, but in this case, it is still not that unreadable.

Answer 2

One option is to do a two-pass replacement, with a token (I like unicode for this, as it's unlikely to appear elsewhere):

var s = 'how((\\s+can|\\s+do)(\\s+i)?)?';
var token = "\u1234";
// Look for the character preceding the ( you want
// to replace. We'll add the token after it.
var patt1 = /([^\\])(?=\()/g;
// The second pattern looks for the token and the (.
// We'll replace both with the desired string.
var patt2 = new RegExp(token + '\\(', 'g');

s = s.replace(patt1, "$1" + token).replace(patt2, "(?:");

console.log(s);

https://jsfiddle.net/48e75wqz/1/