JavaScript regular expression substitution

regexpsub.js

function sub(pattern, string, repl) {
    // Substitute all matches of pattern in string with the value returned by repl
    // given a match and the corresponding group values,
    // similar to Python's re.sub function.
    // Note that pattern must be a "global" RegExp of the form /.../g
    var found;
    var lastIndex = 0;
    var result = "";
    while (found = pattern.exec(string)) {
        var subst = repl.apply(this, found);
        result += string.slice(lastIndex, found.index) + subst;
        lastIndex = found.index + found[0].length;
    }
    result += string.slice(lastIndex);
    return result;
}

var newValue = sub(/(b)(a)/g, "abababc", function(match, group1, group2) {
    return group1 + "d";
});
alert(newValue); // -> "abdbdbc"