Closely associated with the matching variables C<$1>, C<$2>, ... are
the I<backreferences> C<\1>, C<\2>,... Backreferences are simply
matching variables that can be used I<inside> a regexp. This is a
-really nice feature -- what matches later in a regexp is made to depend on
+really nice feature; what matches later in a regexp is made to depend on
what matched earlier in the regexp. Suppose we wanted to look
for doubled words in a text, like 'the the'. The following regexp finds
all 3-letter doubles with a space in between:
print "bad line: '$line'\n";
}
-But this doesn't match -- at least not the way one might expect. Only
+But this doesn't match, at least not the way one might expect. Only
after inserting the interpolated C<$a99a> and looking at the resulting
full text of the regexp is it obvious that the backreferences have
-backfired -- the subexpression C<(\w+)> has snatched number 1 and
+backfired. The subexpression C<(\w+)> has snatched number 1 and
demoted the groups in C<$a99a> by one rank. This can be avoided by
using relative backreferences:
=back
-As we have seen above, Principle 0 overrides the others -- the regexp
+As we have seen above, Principle 0 overrides the others. The regexp
will be matched as early as possible, with the other principles
determining how the regexp matches at that earliest character
position.
# but _does_ print
Hmm. What happened here? If you've been following along, you know that
-the above pattern should be effectively (almost) the same as the last one --
-enclosing the d in a character class isn't going to change what it
+the above pattern should be effectively (almost) the same as the last one;
+enclosing the C<d> in a character class isn't going to change what it
matches. So why does the first not print while the second one does?
The answer lies in the optimizations the regex engine makes. In the first
https://perl5.git.perl.org / perl5.git / blobdiff