.NET正则表达式中的锚点

Question

This is a question about the answer given in the question Check a string to see if all characters are hexadecimal values .

The proposed regular expression is the following:

\A\b[0-9a-fA-F]+\b\Z

Now, \\A and \\Z seem to be the equivalent to ^ and $ respectively. \\Z behaves differently, in that it allows a newline after it when matching (this might or might not be intended).

What I don't understand is why the \\b "match at word boundary" anchor is used. Isn't the beginning/end of a string always a word boundary?

Ultimately, the regex could be rewritten as ^[0-9a-fA-F]$ with the same behavior (ignoring the trailing \\n issue). Am I missing something? Is using \\b required for some weird edge case?

Test cases:

123ABC -> true
123def -> Returns true
123g -> Returns false

Answer 1

The word boundary \\b matches between non-word and word characters, and also at the start of the string if the first character is a word character, and at the end if the last character is a word character.

Thus, \\A\\b[0-9a-fA-F]+\\b\\Z is equal to \\A[0-9a-fA-F]+\\Z because all the characters in the string must be word characters ( [0-9] digits or [a-fA-F] letters) for the pattern to match it .

It would be a different story in this case: \\A\\b[0-9a-fA-F-]+\\b\\Z that would only match strings with word characters at the beginning and end.

Use \\z to match a whole string, with no \\n allowed at the end.