Plural rule parser

plural

/*
This code uses a regular expression to make the json safer.
An potentially safer alternative is https://github.com/joewalnes/filtrex (untested) at the cost of 130Kb.

Can be used for example with this JSON structure:
{
    "_meta_po_header": "nplurals=3; plural=(n==1 ? 0 : n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2);",
    "COLLECTION_CONTENTS[0]": "Kolekcja zawiera %@ przepis.",
    "COLLECTION_CONTENTS[1]": "Kolekcja zawiera %@ przepisy.",
    "COLLECTION_CONTENTS[2]": "Kolekcja zawiera %@ przepisów."
}
*/

var tm;
tm = tm || {};
tm.plural = (function () {
    "use strict";

    var RE_RULE = /^nplurals=\d;\s*plural=(.*);$/, // takes the rule
        RE_STRIP_UNSAFE = /^[n\=\s\?\d\:\%<>\&\(\)\|\!]+$/g; // strips potentially unsafe characters

    return {

        // returns a function that takes a count parameter
        makeRule: function (po_header) {
            var ruleStrMatch,
                safeRuleMatch,
                ruleStr,
                safeRule;
            try {
                ruleStrMatch = po_header.match(RE_RULE);
                if (ruleStrMatch && ruleStrMatch[1]) {
                    ruleStr = ruleStrMatch[1];
                }
                safeRuleMatch = ruleStr.match(RE_STRIP_UNSAFE);
                if (safeRuleMatch && safeRuleMatch[0]) {
                    safeRule = safeRuleMatch[0];
                }
            } catch (e) {
                if (console && console.log) {
                    console.log("PO header contains illegal characters.");
                }
            }
            try {
                if (ruleStr === undefined) {
                    throw "Could not find PO rule string; perhaps PO header contains illegal characters";
                }
                if (safeRule === undefined) {
                    throw "Could not create safe PO rule string; perhaps PO header contains illegal characters";
                }
            } catch (e) {
                if (console && console.log) {
                    console.log("PO header contains illegal characters.");
                }
            }
            return function (n) {
                return eval("+" + safeRule);
            };
        },
        makeKey: function (str, num) {
            return str + "[" + num + "]";
        }
    };
}());