String Substitution Utilities

// Substitute character string c2 for the first c1 in the string aString
function substFirst(aString, c1, c2)
{
	if (aString == "") return aString
	
	// find the character string
	var i = aString.indexOf(c1)
	
	// return original string if not found
	if (i < 0) return aString
	
	// extract the characters before and after the string
	var s1 = aString.substring(0, i)
	var s2 = aString.substring(i+c1.length, aString.length)
	
	// recombine with the new character string and return
	return s1+c2+s2
}

// Substitute character string c2 for the last c1 in the string aString
function substLast(aString, c1, c2)
{
	if (aString == "") return aString
	
	// find the character string
	var i = aString.lastIndexOf(c1)
	
	// return original string if not found
	if (i < 0) return aString
	
	// extract the characters before and after the string
	var s1 = aString.substring(0, i)
	var s2 = aString.substring(i+c1.length, aString.length)
	
	// recombine with the new character string and return
	return s1+c2+s2
}

// Substitute character string c2 for every c1 in aString
function subst(aString, c1, c2)
{
	if (aString == "") return aString
	if (c1 == "") return aString
	
	// avoid infinite recursion when substituting aa for a by
	// providing an offset into the string.
	var argc = subst.arguments.length
	if (argc < 4) {n = 0} else {n = subst.arguments[3]}

	// find the first occurrence of c1 after the threshold
	var i = aString.indexOf(c1, n)
	
	// stop recursion and return the current string when c1 not found
	if (i < 0) return aString
	
	// extract substrings s1 and s2 around the c1
	var s1 = aString.substring(0, i)
	var s2 = aString.substring(i+c1.length, aString.length)
	
	// recurse with this new string
	return subst(s1+c2+s2, c1, c2, (i+c2.length))
}