Find longest repeating substring in JavaScript using regular expressions

Question

I\'d like to find the longest repeating string within a string, implemented in JavaScript and using a regular-expression based approach.

I have an PHP implementation

Answer 1

It seems JS regexes are a bit weird. I don't have a complete answer, but here's what I found.

Although I thought they did the same thing re.exec() and "string".match(re) behave differently. Exec seems to only return the first match it finds, whereas match seems to return all of them (using /g in both cases).

On the other hand, exec seems to work correctly with ?= in the regex whereas match returns all empty strings. Removing the ?= leaves us with

re = /((.+)(?:.*?\2)+)/g

Using that

"XXinputinputYY".match(re);

returns

["XX", "inputinput", "YY"]

whereas

re.exec("XXinputinputYY");

returns

["XX", "XX", "X"]

So at least with match you get inputinput as one of your values. Obviously, this neither pulls out the longest, nor removes the redundancy, but maybe it helps nonetheless.

One other thing, I tested in firebug's console which threw an error about not supporting $1, so maybe there's something in the $ vars worth looking at.