Repeating C# Regex replace

Question

I want to create a small tool to convert some code.

Most of it can be done by Regex, so I do not need an actual parser.

But there is a problem with repeating patterns. Below a simplified sample:

var nl = Environment.NewLine;
var temp = 
@"Other things
End
End
Other things";

temp = Regex.Replace(temp, @"(\r\nEnd\r\n)+",@"" + nl + "}" + nl + "");

This will return

Other things
}
End
Other things

So the second End is not replaced. (except if I execute the same Regex.Replace twice)

Any idea how to solve this?

Best regards

At dotnetperls.com/regex show how to use the regular expression. It is very fast to read and quite interesting !! — raBinn
– raBinn, Commented Feb 22, 2017 at 12:31

xanatos · Accepted Answer · 2017-02-22 12:30:52Z

4

It happens because matches can't overlap. You have

Other things\r\n
End\r\n
End\r\n
Other things\r\n

that is matched as

Other things***From Here***\r\n
End\r\n***To Here***
End\r\n
Other things

The regex can't match the second \r\nEnd\r\n because the prefixed \r\n has already been captured.

Try:

temp = Regex.Replace(temp, @"(?<=\r\n)End(?=\r\n)+", "}");

Using the non-capturing grouping (?<=) and (?=) you can solve the problem (but note that being non-capturing, you can't replace what is matched in the (?<=) and (?=), so the difference in the replaced text "}")

answered Feb 22, 2017 at 12:30

xanatos

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