Regex Conditional Values

Question

If I have a string like the following that can have two possible values (although the value JB37 can be variable)

String One\r\nString Two\r\n
String One\r\nJB37\r\n

And I only want to capture the string if the value following String One\r\n does NOT equal String Two\r\n, how would I code that in Regex?

So normally without any condition, this is what I want: String One\r\n(.+?)\r\n

Wiktor Stribiżew · Accepted Answer · 2018-08-28 16:05:20Z

With regex, you may resort to a negative lookahead:

String One\r\n(?!String Two(?:\r\n|$))(.*?)(?:\r\n|$)

You may also use [^\r\n] instead of .:

String One\r\n(?!String Two(?:\r\n|$))([^\r\n]*)

If you use RegexOptions.Multiline, you will also be able to use

(?m)String One\r\n(?!String Two\r?$)(.*?)\r?$

Details

(?m) - a RegexOptions.Multiline option that makes ^ match start of a line and $ end of line positions
String One\r\n - String One text followed with a CRLF line ending
(?!String Two\r?$) - a negative lookahead that fails the match if immediately to the right of the current location, there is String Two at the end of the line
(.*?) - Capturing group 1: any zero or more chars other than line break chars, as few as possible, up to the leftmost occurrence of
\r?$ - an optional CR and end of the line (note that in a .NET regex, $ matches only in front of LF, not CR, in the multiline mode, thus, \r? is necessary).

C# demo:

var m = Regex.Match(s, @"(?m)String One\r\n(?!String Two\r?$)(.*?)\r?$");
if (m.Success) 
{
     Console.WriteLine(m.Groups[1].Value);
}

If CR can be missing, add ? after each \r in the pattern.

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