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Use the replace() method to replace the first occurrence of a character in a string. The method takes a regular expression and a replacement string as parameters and returns a new string with one or more matches replaced.
count : Maximum number of pattern occurrences to be replaced. The count must always be a positive integer if specified. . By default, the count is set to zero, which means the re. sub() method will replace all pattern occurrences in the target string.
How to use RegEx with . replace in JavaScript. To use RegEx, the first argument of replace will be replaced with regex syntax, for example /regex/ . This syntax serves as a pattern where any parts of the string that match it will be replaced with the new substring.
From MSDN:
Replace(String, String, Int32)
Within a specified input string, replaces a specified maximum number of strings that match a regular expression pattern with a specified replacement string.
Isn't this what you want?
Just to answer the original question... The following regex matches only the first instance of the word foo:
(?<!foo.*)foo
This regex uses the negative lookbehind (?<!) to ensure no instance of foo is found prior to the one being matched.
You were probably using the static method. There is no (String, String, Int32) overload for that. Construct a regex object first and use myRegex.Replace.
In that case you can't use:
string str ="abc546_$defg";
str = Regex.Replace(str,"[^A-Za-z0-9]", "");
Instead you need to declare new Regex instance and use it like this:
string str ="abc546_$defg";
Regex regx = new Regex("[^A-Za-z0-9]");
str = regx.Replace(str,"",1)
Notice the 1, It represents the number of occurrences the replacement should occur.
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