I’ve recently been working in someone’s code that uses Regex quite a bit, and wanted to make a simple observation: I contend that in 99% of the regex usage I’ve seen, the pattern /.*/ is used where /.*?/ is intended.

When Regex processes .*, it shifts in the entire input then backtracks until it finds the pattern that follows your .*. Most of the time, though, the coder clearly is thinking in left-to-right terms. An example:


string input = "<blah>here's some stuff</blah>other>otherstuff</other>";
foreach(string pattern in new string[] {">.*<", ">.*?<"})
{
    Match m = Regex.Match(input, pattern);
    Console.WriteLine(m.Value);
}


The first pattern is the greedy one. It will match “<here’s some stuff>/blah><other>otherstuff<”. The second is the non-greedy version, and it matches “>here’s some stuff<”, which is probably the desired match. But even if you end up matching the right stuff most of the time, the non-greedy form should be more performant when you’re dealing with longer input strings.