Complex Regex problem with hyperlink not matching "?"

Question

I am working in replacing a text with a hyperlink in C#. The problem here is that the link has a question mark

Case 1:No problem

Input: ASAss123

Case 2:Problem here

Output:ASAsq123">ASAsq123

(Note: First occurrence of ASAss123 is hyperlink and is replaced as http://stack.com/temp/test?order=sam&identifier=<a href= and second occurrence is just plain text)

Preferred Output: ASAsq123

How can I rectify this problem. Code here for your reference:

 mailItem.HTMLBody = Regex.Replace(
     mailItem.HTMLBody,
     "(?<!http://stack.com/temp/test?order=sam&identifier=)ASA[a-z][a-z][0-9][0-9][0-9](?!</a>)",
     "<a href=\"http://stack.com/temp/test?order=sam&identifier=$&\">$&</a>");

The problem here is with the "?" found in the second argument. If I get rid this of "?" in both 2nd and 3rd arguments, this works perfectly fine.

But I cannot get rid of "?", because it is needed for the URL to function. How can I solve this problem?

I tried escape sequence with \? and C sharp says escape sequence unrecognized...

I'm sorry but this is not very clear at ALL. Can you please provide an example link which is causing you problems. Just provide the URL, please. — Ian G
– Ian G, Commented Jul 1, 2011 at 16:14
Please hover over the hyperlinks and you will see the respective links for the example. — c_prog_90
– c_prog_90, Commented Jul 1, 2011 at 16:18

agent-j · Accepted Answer · 2011-07-01 16:10:10Z

4

It looks like you need to escape your test? like test\? so it doesn't mean optional t.

answered Jul 1, 2011 at 16:10

agent-j

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2 Comments

c_prog_90 Over a year ago

I tried this C sharp does not allow this. It says Escape Sequence unrecognized.

agent-j Over a year ago

Consider using the @"test\?" syntax instead of the "test\\?" syntax... it's easier with expressions.

Petar Ivanov · Accepted Answer · 2011-07-01 16:43:43Z

3

You need to escape like this:

\\?

You are escaping ? In the regex, but \ needs to also be escaped in the c# string.

answered Jul 1, 2011 at 16:43

Petar Ivanov

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4 Comments

Ssithra Over a year ago

It should work but this solution has a big drawback : it mixes C# escape sequences and regex escape sequences all in one single string (in that case : a C# escape sequence produces a regex one, wow !). The regex are already so complex by themselves that this solution, though correct, would probably increase the confusion and decrease the readability and maintenability.

c_prog_90 Over a year ago

This is an interesting observation!

Petar Ivanov Over a year ago

@Ssithra, so how would you use regex avoiding this problem? This is what regexes are.

Ssithra Over a year ago

@fiver I would keep Thinkcool's regex unchanged and would prefix it by "@" to let the C# compiler know that it should not care about escape sequences in the string if there are some. See my own answer (which was not chosen as the right one, sigh, you win anyway ! ;-))

Ssithra · Accepted Answer · 2011-07-01 16:44:14Z

2

C# doesn't recognize the sequence \? because it is a regex one, not a C# one.

To prevent C# from trying to recognize escape sequences in your string and make C# treat your \? like any other characters, you must prefix your string with @ :

mailItem.HTMLBody = Regex.Replace(
 mailItem.HTMLBody,
 @"(?<!http://stack.com/temp/test?order=sam&identifier=)ASA[a-z][a-z][0-9][0-9][0-9](?!</a>)",
 "<a href=\"http://stack.com/temp/test?order=sam&identifier=$&\">$&</a>");

answered Jul 1, 2011 at 16:44

Ssithra

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