Match whole word (Visual Studio style) - regex

Question

I am trying to add Match Whole Word search to my small application.
I want it to do the same thing that Visual Studio is doing.
So for example, below code should work fine:
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
String input = "[ abc() *abc ]";
Match(input, "abc", 2);
Match(input, "abc()", 1);
Match(input, "*abc", 1);
Match(input, "*abc ", 1);
}
private void Match(String input, String pattern, int expected)
{
String escapedPattern = Regex.Escape(pattern);
MatchCollection mc = Regex.Matches(input, #"\b" + escapedPattern + #"\b", RegexOptions.IgnoreCase);
if (mc.Count != expected)
{
throw new Exception("match whole word isn't working");
}
}
}
Searching for "abc" works fine but other patterns return 0 results.
I think \b is inadequate but i am not sure what to use.
Any help would be appreciated.
Thanks

Answer 1

The \b metacharacter matches on a word-boundary between an alphanumeric and non-alphanumeric character. The strings that end with non-alphanumeric characters end up failing to match since \b is working as expected.
To perform a proper whole word match that supports both types of data you need to:
use \b before or after any alphanumeric character
use \B (capital B) before or after any non-alphanumeric character
not use \B if the first or last character of the pattern is intentionally a non-alphanumeric character, such as your final example with a trailing space
Based on these points you need to have additional logic to check the incoming search term to shape it into the appropriate pattern. \B works in the opposite manner of \b. If you don't use \B then you could incorrectly end up with partial matches. For example, the word foo*abc would incorrectly be matched with a pattern of #"\*abc\b".
To demonstrate:
string input = "[ abc() *abc foo*abc ]";
string[] patterns =
{
#"\babc\b", // 3
#"\babc\(\)\B", // 1
#"\B\*abc\b", // 1, \B prefix ensures whole word match, "foo*abc" not matched
#"\*abc\b", // 2, no \B prefix so it matches "foo*abc"
#"\B\*abc " // 1
};
foreach (var pattern in patterns)
{
Console.WriteLine("Pattern: " + pattern);
var matches = Regex.Matches(input, pattern);
Console.WriteLine("Matches found: " + matches.Count);
foreach (Match match in matches)
{
Console.WriteLine(" " + match.Value);
}
Console.WriteLine();
}

Answer 2

I think this is what you're looking for:
#"(?<!\w)" + escapedPattern + #"(?!\w)"
\b is defined in terms of the presence or absence of "word" characters both before and after the current position. You only care about the what's before the first character and what's after the last one.

Answer 3

The \b is a zero-width assertion that matches between a word character and a non-word character.
Letters, digits and underscores are word characters. *, SPACE, and parens are non-word characters. therefore, when you use \b*abc\b as your pattern, it does not match your input, because * is non-word. Likewise for your pattern involving parens.
To solve this,
You will need to eliminate the \b in cases where your input (unescaped) pattern begins or ends with non-word characters.
public void Run()
{
String input = "[ abc() *abc ]";
Match(input, #"\babc\b", 2);
Match(input, #"\babc\(\)", 1);
Match(input, #"\*abc\b", 1);
Match(input, #"\*abc\b ", 1);
}
private void Match(String input, String pattern, int expected)
{
MatchCollection mc = Regex.Matches(input, pattern, RegexOptions.IgnoreCase);
Console.WriteLine((mc.Count == expected)? "PASS ({0}=={1})" : "FAIL ({0}!={1})",
mc.Count, expected);
}