Get the number of capturing parentheses of a regex pattern in Javascript

find_parens_sub.js

/*
It isn't a full implementation : you can't use it to search named groups or anything.
It just returns the number of capturing parentheses of the pattern passed as the 1st argument.

Usage :
find_parens_sub(/(my pattern)/i.source); // 1

Idea ? comment ? insult ? camille.hodoul [at] gmail.com or @Eartz_HC

I copied this function from http://www.opensource.apple.com/source/pcre/pcre-4.2/pcre/pcre_compile.c ,
but removed the parts I don't need, so it isn't an exact implementation. It should do the trick though.
*/
function find_parens_sub(ptr,count) {
	"use strict"
	var count = count || 0;
	var start_count = count;
	var hwm_count = start_count;
	var i = 0;
	var dup_parens = false;
	/* If the first character is a parenthesis, check on the type of group we are
dealing with. The very first call may not start with a parenthesis. */
	if(ptr[0]=="(") {
		if(ptr[1] == "?" && ptr[2]=="|") {
			i += 3;
			dup_parens = true;
		}

		/* Handle a normal, unnamed capturing parenthesis */
		else if(ptr[1] != "?" && ptr[1] != "*") {
			count += 1;
			i++;
		}
		/* Handle a condition. If it is an assertion, just carry on so that it
		  is processed as normal. If not, skip to the closing parenthesis of the
		  condition (there can't be any nested parens. */
		else if(ptr[i+2]=="(") {
			i+= 2;
			if(ptr[i+1] != "(") {
				while(!!ptr[i] && ptr[i]!=")") i++;
				if(ptr[i]!=0) i++;
			}
		}
		/* We have either (? or (* and not a condition */
		else {
			i+=2;
			if(ptr[i]=="P") i++;

			/* We have to disambiguate (?<! and (?<= from (?<name> for named groups */
			if((ptr[i]=="<" && ptr[i+1] != "!" && ptr[i+1] != "=") || ptr[i]=="'") {
				count++;
			}
		}
	}
	
	/* Past any initial parenthesis handling, scan for parentheses or vertical
	bars. */
	for(;!!ptr[i];i++) {
			/* Skip over backslashed characters and also entire \Q...\E */
			if(ptr[i]=="\\") {
				if(!ptr[++i]) throw new Error("Weird backslash ?");
				if(ptr[i]=="Q") for(;;) {
					while(!!ptr[++i] && ptr[i] != "\\") {};
					if(!ptr[i]) throw new Error("No \E ?");
					if(ptr[++i]=="E") break;
				}
				continue;
			}
			/* Skip over character classes; this logic must be similar to the way they
			  are handled for real. If the first character is '^', skip it. Also, if the
			  first few characters (either before or after ^) are \Q\E or \E we skip them
			  too. */
			if(ptr[i]=="[") {
				var negate_class = false;
				for(;;) {
					var c = ptr[++i];
					if(c=="\\") {
						if(ptr[i] == "E") i++;
						else if(!strncmp(ptr[i+1]),"Q\\E",3) {
							i+=3;
						}
						else {
							break;
						}
					}
					else if(!negate_class && c == "^") {
						negate_class=true;
					}
					else break;
				}


				/* If the next character is ']', it is a data character that must be
				skipped, except in JavaScript compatibility mode. */
				if(ptr[i] == "]" && false) {
					i++;
				}
				while(ptr[++i] != "]") {
					if(!ptr[i]) {
						return count;
					}
					if(ptr[i]=="\\") {
						if(!ptr[++i]) throw new Error("Weird backslash ?");
						if(ptr[i]=="Q")for(;;) {
							while(!!ptr[++i] && ptr[i]!= "\\") {};
							if(!ptr[i]) throw new Error("No \E ?");
							if(ptr[++i]=="E") break;
						}
						continue;
					}
				}
				continue;
			}
		
	
		/* Check for the special metacharacters */
		if(ptr[i] == "(") {
			
			count = find_parens_sub(ptr.slice(i),count);
			return count;
		}
		else if(ptr[i]==")") {
			if(dup_parens && count < hwm_count) count = hwm_count;
		}
		else if(ptr[i] == "|" && dup_parens) {
			if(count > hwm_count) hwm_count = count;
			count = start_count;
		}
	}
	return count;
}

function strncmp(str1, str2, lgth) {
  //       discuss at: http://phpjs.org/functions/strncmp/
  //      original by: Waldo Malqui Silva
  //         input by: Steve Hilder
  //      improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
  //       revised by: gorthaur
  // reimplemented by: Brett Zamir (http://brett-zamir.me)
  //        example 1: strncmp('aaa', 'aab', 2);
  //        returns 1: 0
  //        example 2: strncmp('aaa', 'aab', 3 );
  //        returns 2: -1

  var s1 = (str1 + '')
    .substr(0, lgth);
  var s2 = (str2 + '')
    .substr(0, lgth);

  return ((s1 == s2) ? 0 : ((s1 > s2) ? 1 : -1));
}