Empty Regex never fails but failed?

leszekdubiel has asked for the wisdom of the Perl Monks concerning the following question:

I am completely confused why this fails:

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /alfa/;'
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Die should never happen because empty pattern matches any text... Why? Why? Same on 5.24 and 5.26. Linux Debian. The code

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /al/;'
[download]

also fails, but this one not (!!!!):

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /a/;'
[download]

Comment on Empty Regex never fails but failed? Select or Download Code

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Re: Empty Regex never fails but failed? by haukex (Archbishop) on Feb 12, 2018 at 23:22 UTC
See Regexp Quote Like Operators: The empty pattern `//` If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the `g` and `c` flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match). That's why your third example works: `/a/` matches in both `alfa` and `whatever`. `$ printf "Foo\nBar\nQuz\nBaz" \| perl -nle 'print // ?"yes":"no"; /a/' yes yes no yes` [download] (See also Repeated Patterns Matching a Zero length Substring, although I think that's less relevant in this case.)	[reply] [d/l] [select]

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Re: Empty Regex never fails but failed?
by haukex (Archbishop) on Feb 12, 2018 at 23:22 UTC

Regexp Quote Like Operators

The empty pattern //

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the g and c flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match).

That's why your third example works: /a/ matches in both alfa and whatever.

$ printf "Foo\nBar\nQuz\nBaz" | perl -nle 'print // ?"yes":"no"; /a/'
yes
yes
no
yes
[download]

(See also Repeated Patterns Matching a Zero length Substring, although I think that's less relevant in this case.)

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