\b doesn't match when the preceding character is a word boundary

Question

I have a rather peculiar problem. I'm trying to find a pattern like [some string][word boundary]. Simplified, my code is:

final Pattern pattern = Pattern.compile(Pattern.quote(someString) + "\\b");
final String value = someString + " ";
System.out.println(pattern.matcher(value).find());

My logic tells me this should always output true, regardless of what someString is. However:

• if someString ends with a word character (e.g. "abc"), true is outputted;
• if someString ends with a word boundary (e.g. "abc."), false is outputted.

Any ideas what is happening? My current workaround is to use \W instead of \b, but I'm not sure of the implications.

Answer 1

A dot then a space is not a word boundary.

A word boundary is between a word character, then a non-word character, or visa versa.
ie between [a-zA-Z0-9_][^a-zA-Z0-9_] or [^a-zA-Z0-9_][a-zA-Z0-9_]

Answer 2

A word boundary is a non-word character that is preceded by a word character or vice versa. The space preceded by a period (2 non-word characters) does not meet this requirement.

The effect of using \W is that any non-word characters will be matched (the same as \b, but without the condition that the character is preceded by a word character), which seems correct for your example.