When and why did the output of qr() change?

Question

The output of perl's qr has changed, apparently sometime between versions 5.10.1 and 5.14.2, and the change is not documented--at least not fully.

To demonstrate the change, execute the following one-liner on each version:

perl -e 'print qr(foo)is."\n"'

Output from perl 5.10.1-17squeeze6 (Debian squeeze):

(?-xism:foo)

Output from perl 5.14.2-21+deb7u1 (Debian wheezy):

(?^:foo)

The perl documentation (perldoc perlop) says:

$rex = qr/my.STRING/is;
print $rex; # prints (?si-xm:my.STRING)
s/$rex/foo/;

which appears to no longer be true:

$ perl -e 'print qr/my.STRING/is."\n"'
(?^si:my.STRING)

I would like to know when this change occurred (which version of Perl, or supporting library or whatever).

Some background, in case it's relevant:

This change has caused a bunch of unit tests to fail. I need to decide if I should simply update the unit tests to reflect the new format, or make the tests dynamic enough to support both formats, etc. To make an informed decision, I would like to understand why the change took place. Knowing when and where it took place seems like the best place to start in that investigation.

Answer 1

It's documented in perl5140delta:

Regular Expressions

(?^...) construct signifies default modifiers

[...] Stringification of regular expressions now uses this notation. [...]

This change is likely to break code that compares stringified regular expressions with fixed strings containing ?-xism.

The function regexp_pattern can be used to parse the modifiers for normalisation purposes.