>> You're supposed to use the \1 format to match a current match,
>> like a duplicated word
>> $ echo "her here hear hear hop hip hip ho!" | perl \
-pe 's/(\w+)\s+\1\s+/double "${1}s" /g;'
Barry B wrote:
> I am confused about this. I thought that a back-reference looks like
"$1", not "\1". Is there a difference?
Yeah, mostly w/ placement. Back refs on the left hand side (LHS) of the
subst:
s/(\w+)\s+\1\s+/
are backslash digit. Backreferences to the captured match on the RHS use
$1 as they do outside the subst command. This got made more concrete
somewhere in early v5, I believe. As noted, warnings will tell you:
\1 better written as $1 at -e line 1
if you had tried:
$ echo "her here hear hear hop hip hip ho!" | perl \
-w -pe 's/(\w+)\s+\1\s+/double "\1s" /g;'
though it still works. But the idea is the \1 version can be used during
the course of the matching phase, but $1 version is used during the
replacement phase. In a sense, the \1 'magic var' is supposed to be
localized to the LHS:
s/ ... /
context, while $1 et alia are actual globals so you can do:
> -pe 'if ( s/(\w+)\s+\1\s+/double "${1}s" / ) { warn "found a $1\n"; }'
and have a value outside the subst command. Trying "\1" in the warn():
warn "found a \1\n";
would get you ... well you get the "001" char ;->
$ echo "her here hear hear hop hip hip ho" | perl -pe 'if (
s/(\w+)\s+\1\s+/double "${1}s" / ) { warn "found a $1\n" };'
found a hear
her here double "hears" hop hip hip ho
but (note, I dropped the "/g"):
$ echo "her here hear hear hop hip hip ho" | perl -pe 'if (
s/(\w+)\s+\1\s+/double "${1}s" / ) { warn "found a \1\n" };'
found a <unprintable>
her here double "hears" hop hip hip ho
Interesting, in a way, is how with the '/g' you get:
echo "her here hear hear hop hip hip ho" | perl -pe 'if (
s/(\w+)\s+\1\s+/double "${1}s" /g ) { warn "found a $1\n" };'
found a h
her here double "hears" hop double "hips" ho
I think what happens here is the capture parens matched the 'h' of the
final 'ho' but there's no match for the \1 part. So no subst is done.
However, $1 keeps the captured value (it did match a \w+ char). Not
exactly what I expected, to be honest - I'd've thought if the LHS RE
failed, $1 wouldn't be 'updated' but would keep the last full match (i.e.
'hip').
Wrong again ...
a
----------------------
Andy Bach
Systems Mangler
Internet: andy_b...@wiwb.uscourts.gov
Voice: (608) 261-5738;
Cell: (608) 658-1890
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which we can perform without thinking about them.
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