Regular Expression - Boundary Matcher

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Example

Word boundary

word boundary

Case 1:

• The regex: \bdog\b
• Input String to search: The dog plays in the yard
• Result: Found the text dog starting at index 4 and ending at index 7.

Case 2:

• The regex: \bdog\b
• Input string to search: The doggie plays in the yard.
• No match found.

Non-word boundary

A non-word boundary is \B.

Example 1:

• The regex: \bdog\B
• Input string to search: The dog plays in the yard.
• No match found.

Example 2:

• The regex: \bdog\B
• Input string to search: The doggie plays in the yard.
• I found the text dog starting at index 4 and ending at index 7.

End of the previous match

To require the match to occur only at the end of the previous match, use \G:

Example 1:

• The regex: dog
• Input string to search: dog dog
• Result:
  • I found the text “dog” starting at index 0 and ending at index 3.
  • I found the text “dog” starting at index 4 and ending at index 7.

Example 2:

• The regexp: \Gdog
• Input string to search: dog dog
• Result: I found the text dog starting at index 0 and ending at index 3.

The second example finds only one match, because the second occurrence of dog does not start at the end of the previous match.

Documentation / Reference

• http://docs.oracle.com/javase/tutorial/essential/regex/bounds.html