hmm, strange in 5.10 this uncommon feature seems to be buggy:

$ perl
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
match 2
[download]

while B::Deparse claims both lines to be identical ...

perl -MO=Deparse 
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
if ('_$x_' =~ /\$x/) {
    print 'match 1';
}
if ('_$x_' =~ /\$x/) {
    print 'match 2';
}
__DATA__
- syntax OK
[download]

... does B::Concise show that the interpolation is not inhibited

perl -MO=Concise
if ( '_$x_' =~ '$x') { print "match 1" }                      
if ( '_$x_' =~ /\$x/){ print "match 2" }
__DATA__
g  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 3 -:1) v:{ ->3
-     <1> null vKP/1 ->9
5        <|> and(other->6) vK/1 ->9
4           </> match(/"$x"/) sKS/RTIME ->5              <-- INTERPOLA
+TED
3              <$> const[PV "_$x_"] s ->4
-           <@> scope vK ->-
-              <0> ex-nextstate v ->6
8              <@> print vK ->9
6                 <0> pushmark s ->7
7                 <$> const[PV "match 1"] s ->8
9     <;> nextstate(main 5 -:2) v:{ ->a
-     <1> null vKP/1 ->g
c        <|> and(other->d) vK/1 ->g
b           </> match(/"\\$x"/) sKS/RTIME ->c            <-- NO INTERP
+OLATION
a              <$> const[PV "_$x_"] s ->b
-           <@> scope vK ->-
-              <0> ex-nextstate v ->d
f              <@> print vK ->g
d                 <0> pushmark s ->e
e                 <$> const[PV "match 2"] s ->f
- syntax OK
[download]

added marks to highlight differences.

Can someone please check for other perl versions?

Bug in Deparse. Still present.

The regex pattern is

$x         # End of line followed by "x"
[download]

so the literal should be

m'$x'
[download]

It's the only match operator that will produce exactly that regex pattern.

shorter example

interpolation, the \Q\E is interpolated in double quoted strings

$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m{\Qa??\E}; "
Compiling REx "a\?\?"
Final program:
   1: EXACT <a??> (3)
   3: END (0)
anchored "a??" at 0 (checking anchored isall) minlen 3
Guessing start of match in sv for REx "a\?\?" against "a??q"
Found anchored substr "a??" at offset 0...
Guessed: match at offset 0
Freeing REx: "a\?\?"
[download]

use qr to see how \Q\E gets interpolated

$ perl -le " print qr{\Qa??\E}; "
(?^:a\?\?)
[download]

without interpolation the pattern is completely different

$ perl -le " print qr'\Qa??\E'; "
(?^:\Qa??\E)
[download]

The regex metacharcters are still regex metacharacters (pattern)

$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m'\Qa??\E'; "
Compiling REx "\Qa??\E"
Final program:
   1: EXACT <Q> (3)
   3: MINMOD (4)
   4: CURLY {0,1} (8)
   6:   EXACT <a> (0)
   8: EXACT <E> (10)
  10: END (0)
anchored "Q" at 0 floating "E" at 1..2 (checking floating) minlen 2
Guessing start of match in sv for REx "\Qa??\E" against "a??q"
Did not find floating substr "E"...
Match rejected by optimizer
Freeing REx: "\Qa??\E"
[download]

perlrebackslash :)

Probably you are right, I'm, still too decaffeinated to tell! ;-)

But then B::Deparse is buggy.

Cheers Rolf

( addicted to the Perl Programming Language)

Um, $ is still a metacharacter, try '\$x'

> Um, $ is still a metacharacter, try '\$x'

If "'" is the delimiter, no interpolation is performed on the PATTERN.

Cheers Rolf

( addicted to the Perl Programming Language)