The technical explanation for this is given in comment 3 of the page and sums it up perfectly: "I think the flaw here is that HTML is a Chomsky Type 2 grammar

The technical explanation for this is given in comment 3 of the page and sums it up perfectly: "I think the flaw here is that HTML is a Chomsky Type 2 grammar (context free grammar) and RegEx is a Chomsky Type 3 grammar (regular expression). Since a Type 2 grammar is fundamentally more complex than a Type 3 grammar - you can't possibly hope to make this work. But many will try, some will claim success and others will find the fault and totally mess you up." More info: http://en.wikipedia.org/wiki/Chomsky_hierarchy http://en.wikipedia.org/wiki/Chomsky_hierarchy