Regex Word Boundary

Summary: in this tutorial, you will learn about word boundaries in regular expressions to match a position between a word character and a non-word character.

Introduction to Regex word boundary

A word boundary defines a position that denotes the boundary between a word character and a non-word character.

The following defines three positions in a string that are word boundaries:

1. At the start of the string, if the first character is a word character. (criterion #1)
2. At the end of the string, if the last character is a word character. (criterion #2)
3. Between a word character and a non-word character. (criterion #3)

For example, the word boundary positions in the string “C# 11” are as follows:

• Before the letter “C” (criterion #1).
• After the “#” symbol (criterion #3).
• After the space character between “C#” and “11”. (criterion #3)
• After the last character “1” (criterion #2).

Regular expressions use \b metacharacter to denote a word boundary. The \b metacharacter matches a word boundary in a string and does not match any actual character.

For example, the following pattern uses the \b word boundary to match the whole word in a string:

\bword\bCode language: C# (cs)

The following example finds the matches that contain the word sea:

using System.Text.RegularExpressions;
using static System.Console;


var text = "She loves the sea and seaside";
var pattern = "sea";

var matches = Regex.Matches(text, pattern);
foreach (var match in matches)
{
    WriteLine(match);
}Code language: C# (cs)

It returns both sea and seaside.

To match the whole word sea, you can use the word boundary \b like this:

using System.Text.RegularExpressions;
using static System.Console;


var text = "She loves the sea and seaside";
var pattern = @"\bsea\b";

var matches = Regex.Matches(text, pattern);
foreach (var match in matches)
{
    WriteLine(match);
}Code language: C# (cs)

Output:

seaCode language: C# (cs)

Now, it returns one match instead of two.

Summary

• Use \b metacharacter to match a word boundary in a string.
• Use \bword\b to match the whole word.