How to let regex know that it should stop after a first occurence?

Question

string time = "Job started: donderdag 6 mei 2010 at 20:00:02"
var filterReg = new Regex(@".*:", RegexOptions.Compiled);
time = filterReg.Replace(time, String.Empty);

Is it possible to stop after the first occurence? so at the first ":".

Answer 1

By using a more specific regex

new Regex(@"^[^:]*:", RegexOptions.Compiled);

Your .*: does this

1. .* matches everything, greedily, so it runs right to the end of the string.
2. : tries to match, so the regex engine goes back one character at a time (this is called backtracking) to find a match. It stops at the first colon it finds (seen from the end of the string)

whereas ^[^:]*: does this:

1. ^ anchors the regex to the start of the string. no matches in the middle of the string can occur.
2. [^:]* matches everything except colons, greedily, so it runs right to the first colon
3. : can match easily, because the next character happens to be a colon. Done.

No backtracking involved, this means it is also more efficient.

Answer 2

Use this regex .*?: with the Replace overload :

string time = "Job started: donderdag 6 mei 2010 at 20:00:02"
var filterReg = new Regex(@".*?:", RegexOptions.Compiled);
filterReg.Replace(time, String.Empty, 1);

Answer 3

Any reason you're using regular expressions to get a simple substring?

time = time.Substring(time.IndexOf(":") + 1);