C#. Fastest Regex to match any string

Quite simple. What's the fastest regex that will return true for any input?

Edit: why the down votes? Seems like a very legitimate question. I use a function that takes a regex filter and I want to know what will make it match everything the most quickly. "Not using a regex" is not an answer开发者_JAVA技巧.

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class FastestRegex
{
    public static readonly Regex RE = new Regex("", RegexOptions.Compiled);
}

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I would say that it would have to be something like

.*?

which would check for a match with any character, but because of the lazy operator, would match no string at all. I imagine that it would return before even checking the first character. This is assuming that the empty string doesn't return true for all inputs.

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I'd guess that either the empty string, "", or the start-of-input, "^", would result in the fastest (positive) match for any string.

You should try it yourself though: running a few quick tests, the pattern "^" is usually faster than "" on Oracle's 1.6 JRE, but on Mono 2.4, it's the other way around. In both cases, .*? is way slower.

But again: test things on your own system.

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There is no fastest regex that never runs a regex. Therefore the fastest regex is the one that actually runs. /[\S\s]?/ or, I'm guessing this is faster /.?/s

Addon - Interresting though /^/ benched fractionally faster in perl looped 30 million times. However, pre-compiling all the regex' first slows them all down by a factor of 5. Go figure. Maybe because its too simple an expression.