Also, what characters does the "\b" assertion encompass?

It doesn't encompass any characters. It is zero-width and matches the break between word and non-word characters. In your example, it is completely superfluous. You could just as easily have used /\w+/g instead.

If you want to include other characters, you can just use your own character class. You stated outright that you want to include a slash ("/"), but one of your examples, "4-5/16", indicates you'd like a dash too. So, try m![-/\w]+!g and see if that does the trick.

-sauoq
"My two cents aren't worth a dime.";

/(?: (?<!\w)(?=\w) | (?<=\w)(?!\w) )/x

That means that you can create your own version using zero-width lookahead and lookbehind assertions along with character classes.

You may have a look at the entire Why do zero width assertions care about lookahead/behind? thread for further discussion on the \b metacharacter and zero-width assertions.

What I'd like to do is redefine what "\b" is.

You can, by overloading regular expressions. The following is just a quick hack I threw together, but I think it catches most cases, including /[\b]/ and /\\b/.

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

BEGIN {
    my $B = '(?:(?<=[\w/])(?=[^\w/])|'  .
               '(?<=[^\w/])(?=[\w/])|'  .
              '^(?=[\w/])|(?<=[\w/])$)';
    $^H |= 0x30000;
    $^H {qr} = sub {
        local $_ = $_ [1];
        s/(\[\^?]?[^]]*]|[^\\]+|\\.)/$1 eq '\b' ? $B : $1/eg;
        $_;
    }
}


while (<DATA>) {
    chomp;
    print "$_: ";
    print $& if /\b\w+\b/;
    print "\n";
}

__DATA__
foo
&foo&
&foo/bar&


foo: foo
&foo&: foo
&foo/bar&:
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Abigail

Please don't do that, unless you are planning on sticking with the same (tested) perl version forever. perldoc perlvar says:

$^H WARNING: This variable is strictly for internal use only. Its availability, behavior, and contents are subject to change without notice.

and

%^H WARNING: This variable is strictly for internal use only. Its availability, behavior, and contents are subject to change without notice.

See perldoc perlre "Creating custom RE engines" for the documented way to overload regular expressions.

\w matches alphanumerics and underscore. \b is effectively the same as using lookbehinds and lookaheads like this:

(?:(?<=\w)(?=\W|\z)|(?:(?<=\W)|(?<=\A))(?=\w)
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Update: Hmm, or even nicer, as merlyn posted in •Re: Why do zero width assertions care about lookahead/behind? (code examples also updated),

(?:(?<!\w)(?=\w)|(?<=\w)(?!\w))
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So to make a specialized version of \b that views "-" and "/" as "word characters" (sort of), you might use something like this:

(?:(?<![\w/-])(?=[\w/-])|(?<=[\w/-])(?![\w/-]))
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So maybe something like this will suit you?

my $w = '\w/-';
my $b = "(?:(?<![$w])(?=[$w])|(?<=[$w])(?![$w]))";
my @words = ($rec =~ /${b}[$w]+${b}/g);
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I've tested this a little but not a lot, and it seems all right. You'll want to verify it yourself before you go using it for anything important :-)

Now assuming you want to actually match "a/c" in the case of your example, you are also changing the meaning of \w, and your new \b still isn't used, so you can just as well write:

while ($rec =~ m![\w/]+!g) { $word = $&; }
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my @words = split /[\s,.]+/,$rec;
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You can use overload to change the meaning of regexps. It works like this: you can redefine the meaning of //, s///, qr//... literals so that perl applies some to them before compiling to internal form. This way, you can change \b's to some look{ahead,behind} expression, or better still, use some other escape sequence for that.

I have no example code for this. I am sure I have once seen one, but I can't find it.

Update: as ysth said, that code is in the perlre pod. Sorry.

Abigail already posted it in the raw, overload.pm-less fashion.