FAQ 6.17 Why don't word-boundary searches with "\b" work for me?

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6.17: Why don't word-boundary searches with "\b" work for me?

    (contributed by brian d foy)

    Ensure that you know what \b really does: it's the boundary between a
    word character, \w, and something that isn't a word character. That
    thing that isn't a word character might be \W, but it can also be the
    start or end of the string.

    It's not (not!) the boundary between whitespace and non-whitespace, and
    it's not the stuff between words we use to create sentences.

    In regex speak, a word boundary (\b) is a "zero width assertion",
    meaning that it doesn't represent a character in the string, but a
    condition at a certain position.

    For the regular expression, /\bPerl\b/, there has to be a word boundary
    before the "P" and after the "l". As long as something other than a word
    character precedes the "P" and succeeds the "l", the pattern will match.
    These strings match /\bPerl\b/.

            "Perl"    # no word char before P or after l
            "Perl "   # same as previous (space is not a word char)
            "'Perl'"  # the ' char is not a word char
            "Perl's"  # no word char before P, non-word char after "l"

    These strings do not match /\bPerl\b/.

            "Perl_"   # _ is a word char!
            "Perler"  # no word char before P, but one after l

    You don't have to use \b to match words though. You can look for
    non-word characters surrounded by word characters. These strings match
    the pattern /\b'\b/.

            "don't"   # the ' char is surrounded by "n" and "t"
            "qep'a'"  # the ' char is surrounded by "p" and "a"

    These strings do not match /\b'\b/.

            "foo'"    # there is no word char after non-word '

    You can also use the complement of \b, \B, to specify that there should
    not be a word boundary.

    In the pattern /\Bam\B/, there must be a word character before the "a"
    and after the "m". These patterns match /\Bam\B/:

            "llama"   # "am" surrounded by word chars
            "Samuel"  # same

    These strings do not match /\Bam\B/

            "Sam"      # no word boundary before "a", but one after "m"
            "I am Sam" # "am" surrounded by non-word chars


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