Regex word boundary expressions

Question

Say for example I have the following string "one two(three) (three) four five" and I want to replace "(three)" with "(four)" but not within words. How would I do it?

Basically I want to do a regex replace and end up with the following string:

"one two(three) (four) four five"

I have tried the following regex but it doesn't work:

@"\b\(three\)\b"

Basically I am writing some search and replace code and am giving the user the usual options to match case, match whole word etc. In this instance the user has chosen to match whole words but I don't know what the text being searched for will be.

Answer 1

Why regex? Simply replace "(three)" with "(four)". Are there any other cases than you mentioned which would make use of regex necessary?

Answer 2

As Gopi said, but (theoretically) catching only (three) not two(three):

string input = "one two(three) (three) four five";

string output = input.Replace(" (three) ", " (four) ");

When I test that, I get: "one two(three) (four) four five" Just remember that white-space is a string character, too, so it can also be replaced. If I did this:

//use same input
string output = input.Replace(" ", ";");

I'd get one;two(three);(three);four;five"

Answer 3

Your problem stems from a misunderstanding of what \b actually means. Admittedly, it is not obvious.

The reason \b\(three\)\b doesn’t match the threes in your input string is the following:

• \b means: the boundary between a word character and a non-word character.
• Letters (e.g. a-z) are considered word characters.
• Punctuation marks such as ( are considered non-word characters.

Here is your input string again, stretched out a bit, and I’ve marked the places where \b matches:

 o n e   t w o ( t h r e e )   ( t h r e e )   f o u r   f i v e
↑     ↑ ↑     ↑ ↑         ↑     ↑         ↑   ↑       ↑ ↑       ↑

As you can see here, there is a \b between “two” and “(three)”, but not before the second “(three)”.

The moral of the story? “Whole-word search” doesn’t really make much sense if what you’re searching for is not just a word (a string of letters). Since you have punctuation characters (parentheses) in your search string, it is not as such a “word”. If you searched for a word consisting only of word characters, then \b would do what you expect.

You can, of course, use a different Regex to match the string only if it surrounded by spaces or occurs at the beginning or end of the string:

(^|\s)\(three\)(\s|$)

However, the problem with this is, of course, that if you search for “three” (without the parentheses), it won’t find the one in “(three)” because it doesn’t have spaces around it, even though it is actually a whole word.

I think most text editors (including Visual Studio) will use \b only if your search string actually starts and/or ends with a word character:

var pattern = Regex.Escape(searchString);
if (Regex.IsMatch(searchString, @"^\w"))
    pattern = @"\b" + pattern;
if (Regex.IsMatch(searchString, @"\w$"))
    pattern = pattern + @"\b";

That way they will find “(three)” even if you select “whole words only”.