Regex instantiation

When matching a string against a regular expression, you have a few choices. You can first instantiate a Regex object with the regular expression and then call the IsMatch method on that object, passing in the string. Also, you can call the static Regex.IsMatch, which takes both the regular expression and the string.

If you repeat the same match many times, you will save CPU cycles by instantiating the Regex object outside the loop, rather than using the static version of IsMatch.

Here is a loop using the static IsMatch:

for (int i = 0; i < 1000; i++)
{
bool b = (Regex.IsMatch("The number 1000.", @"\d+"));
}

And here its counterpart which instantiates Regex outside the loop:

Regex regex = new Regex(@"\d+");
for (int i = 0; i < 1000; i++)
{
bool b = regex.IsMatch("The number 1000.");
}

Here are the results of a typical run:

Test	Time taken (in ticks)
1000 * static IsMatch	21783
1000 * instantiated Regex	14167

If you spend a lot of time matching regular expressions...