if i have a string like this "Hello - World - Hello World"
I want to replace the characters PRECEDING the FIRST instance of the substring " - "
e.g. so replacing the above with "SUPERDOOPER" would leave: "SUPERDOOPER - World - Hello World"
So far I got this: "^[^-]* - "
But this INCLUDES the " - " which is wrong.
how to do this with regex please?
Use a non-capturing group, which looks ahead of the pattern to verify the match, but does not include those characters in the match itself.
(^[^-]*)(?: -)
Edit: after thinking about it again, that seems a little redundant. Wouldn't this work?:
^[^-]*
Gets all non-dash characters between the beginning of the string and continues until it hits a dash? Or do you need to exclude the space as well? If so, go with the first one.
Why do you think that you need to use a regular expression for a string operation like this?
This is simpler and more efficent:
str = "SUPERDOOPER" + str.Substring(str.IndexOf(" -"));
Regex.Replace(input, @"(Hello)( -.*\1)", @"SUPERDOOPER$2");
The pattern: ([^-]+) (- .+)
First expression matches any series of chars not including a dash. Then there's a space. Second expression starts with a dash and a space, followed by a series of one or more "any" chars.
The replacement: "Superdooper $2"
delivers Superdooper - World - Hello World
You can try using regex with positive lookahead: "^[^-]*(?= - )". As far as I know C# supports it. This regex will match exactly what you want. You can find out more about lookahead, look-behind and other advanced regex techniques in famous book "Mastering Regular Expressions".
