如何仅使用.Net Regex.Replace替换第一个匹配项

Question

Regex.Replace("a b c a b c", "a", "*")

returns

"* b c * b c"

Is there a simple way to replace only the first occurence, such that the result is

"* b c a b c"

Cannot find a RegExOption that would limit the replacement. In particular

Regex.Replace("a b c a b c", "a", "*", RegexOptions.Multiline)
Regex.Replace("a b c a b c", "a", "*", RegexOptions.Singleline)

do not make any difference - obviously. There are not much more options however.

Answer 1

Use the appropriate overload

Regex rgx = new Regex("a");
string result = rgx.Replace("a b c a b c", "*", 1); // * b c a b c

Answer 2

Use capturing group along with the starting anchor.

Regex.Replace("a b c a b c", "^([^a]*)a", "$1*")

DEMO

^([^a]*) captures all the characters which exists before the first a . And the following a on the above regex matches only the first a . So by replacing the matched chars with the characters present inside the group index 1 plus * will give you the desired output.