Regex to replace string

Question

I tried to replace   elements in a string using .NET regex - with no luck :)

Assume the following string:

 AA A  C D   A Some Text   here

The rules

1. Do not replace at beginning of line
2. Do only replace single occurrences
3. Do not replace if a space is before or after it (optional)

The desired result from above is (# as replacement character):

 AA#A  C#D   A#Some Text   here

Answer 1

I am not familiar with C#'s particular regex flavor but in PERL/PHP style this works for me:

s/(?<!\A| |&nbsp;)&nbsp;(?!&nbsp;| )/#/g

This relies on negative lookbehind, negative lookahead and the \A = start of input escape sequence.

Answer 2

string s = Regex.Replace(original, "(?<!(&nbsp;| |^))&nbsp;(?!(&nbsp;| ))", "#");

Answer 3

http://msdn.microsoft.com/en-us/library/system.text.regularexpressions.regex.replace.aspx

Answer 4

You may use this one

[^^\s(nbsp;)](nbsp;)[^$\s(nbsp;)]

Answer 5

This should cover all 3 of your requirements. Excuse the formatting; I had to back-tick the first few lines for the   to show up properly.

string pattern = @"(?<!^|&nbsp;)((?<!\s)&nbsp;(?!\s))(?!\1)";

string[] inputs = { " AA A  C D   A Some Text   here", // original

" AA A  C D   A  Some Text   here" // space before/after

};

foreach (string input in inputs)
{
    string result = Regex.Replace(input, pattern, "#");
    Console.WriteLine("Original: {0}\nResult: {1}", input, result);
}

Output:

Original:  AA A  C D   A Some Text   here

Result:  AA#A  C#D   A#Some Text   here

Original:  AA A  C D   A  Some Text   here

Result:  AA#A  C#D   A  Some Text   here