Format a string by expanding values mapped with String values in a nested Object.

expand.js

/**
 * Expand a string using a supplied regex to denote the properties and a
 * possibly nested object to supply the values.
 * 
 * inputString - The string to be formatted.
 * valuesObject - An Object holding either values stored as strings or other Objects
 * nestSeparator - The marker used to indicated a nested property
 * propRegex - Regex used to indicate the property in the inputString. Assumes the actual
 *     key value will be indicated by using ().
 * 
 * Example:
 * expand("A message from afar reads: \"[test.input] [test.input2][pun]\"",
 *     {test: {input: "Hello", input2: "World"}, pun: "!"},
 *     ".", /\[(.*?)\]/g));
 * Output:
 * A message from afar reads: "Hello World!"
 */
function expand(inputString, valuesObject, nestSeparator, propRegex) {
    function fetchValue(match, nestedProp) {
        var propArray = nestedProp.split(nestSeparator),
            value = valuesObject,
            prop;
        
        for (var i = 0; i < propArray.length; i++) {
            prop = propArray[i];
            if (!value[prop]) {
                // If property is false-y, supply value as empty string.                
                value = "";
                break;
            }
            value = value[prop];
        }
        
        return value;
    }
    inputString = inputString.replace(propRegex, fetchValue);
    
    return inputString;
}