Generating the shortest regex to match an arbitrary word list

Here's what I came up with (JavaScript). It turned a list of 20,000 6-digit numbers into a 60,000-character regular expression. Compared to a naive (word1|word2|...) construction, that's almost 60% "compression" by character count.

I'm leaving the question open, as there's still a lot of room for improvement and I'm holding out hope that there might be a better tool out there.

var list = new listChar("");

function listChar(s, p) {
    this.char = s;
    this.depth = 0;
    this.parent = p;
    this.add = function(n) {
        if (!this.subList) {
            this.subList = {};
            this.increaseDepth();
        }
        if (!this.subList[n]) {
            this.subList[n] = new listChar(n, this);
        }
        return this.subList[n];
    }
    this.toString = function() {
        var ret = "";
        var subVals = [];
        if (this.depth >=1) {
            for (var i in this.subList) {
                subVals[subVals.length] = this.subList[i].toString();
            }
        }
        if (this.depth === 1 && subVals.length > 1) {
            ret = "[" + subVals.join("") + "]";
        } else if (this.depth === 1 && subVals.length === 1) {
            ret = subVals[0];
        } else if (this.depth > 1) {
            ret = "(" + subVals.join("|") + ")";
        }
        return this.char + ret;
    }
    this.increaseDepth = function() {
        this.depth++;
        if (this.parent) {
            this.parent.increaseDepth();
        }
    }
}

function wordList(input) {
    var listStep = list;
    while (input.length > 0) {
        var c = input.charAt(0);
        listStep = listStep.add(c);
        input = input.substring(1);
    }
}
words = [/* WORDS GO HERE*/];
for (var i = 0; i < words.length; i++) {
    wordList(words[i]);
}

document.write(list.toString());

Using

words = ["1231","1233","1234","1236","1238","1247","1256","1258","1259"];