Dear brothers and sisters in Perl.

The perlop says:

If the pattern evaluates to the empty string, the last successfully executed regular expression is used instead.

This seems to be true in a simple example: 

$ perl -wle '"a" =~ /a/; // and print for qw( a b a b a b )'
a
a
a

[download]
The first match is successful, and all the a's that match the same regex are printed.

It behaves correctly when combined with the flip flop operator, too, making it possible to treat the boundary lines specially: 

$ perl -le 'print for a .. z' | perl  -nle 'if (/d/ .. /h/) { print un
+less // }'
e
f
g

[download]

It doesn't work as expected when combined with next, though: 

$ perl -le 'print for a .. z' | perl  -nle 'if (/d/ .. /h/) { next unl
+ess //; print }'
d
f
g
h

[download]

Why is "f" printed?

When combined with -Mre=debug, it shows 

...
Matching REx "d" against "d"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave 0
  Found anchored substr "d" at offset 0 (rx_origin now 0)...
  (multiline anchor test skipped)
Intuit: Successfully guessed: match at offset 0
Matching REx "h" against "d"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave -1
  Did not find anchored substr "h"...
Match rejected by optimizer
Matching REx "d" against "d"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave 0
  Found anchored substr "d" at offset 0 (rx_origin now 0)...
  (multiline anchor test skipped)
Intuit: Successfully guessed: match at offset 0
Matching REx "h" against "e"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave -1
  Did not find anchored substr "h"...
Match rejected by optimizer
Matching REx "d" against "e"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave -1
  Did not find anchored substr "d"...
Match rejected by optimizer
Matching REx "h" against "f"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..1] gave -1
  Did not find anchored substr "h"...
Match rejected by optimizer
Matching REx "" against "f"                      (*)
   0 <> <f>                  |   0| 1:NOTHING(2)
   0 <> <f>                  |   0| 2:END(0)
Match successful!
...

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Why is there the empty regex (see (*))? Is the magic of // somehow broken by next? Is this the expected behaviour and is it documented anywhere?

Update: When combined with continue, the output changes. 

$ perl -le 'print for a .. z' | perl  -ne 'if (/d/ .. /h/) { next unle
+ss //; print }} continue {'
d
e
f
g
h

[download]
Note that the continue part is empty!

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

In reply to Empty pattern in regex by choroba

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