Re^2: Strange Regex Behavior

[BrowserUk]

So, it did the second pattern match first, and interpreted $1 to be the result of that pattern match

Then why does quoting $1 'fix' it?

[quester]

AFAIK, the order in which these subexpressions are evaluated is not defined. Adding one more operation (more or less any operation, "" or - or sqrt all work) does change the order of evaluation. But in the absence of some rule requiring the second and third operands of ?: to be evaluated after the first one rather than before, that's merely a detail of the implementation. I don't see any rule about it offhand in "conditional operator" in perlop.

[BrowserUk]

Hm. I believe the fact that the entire ternary expression is wrapped in parens means that the contents of those parens must be evaluated before anything outside them per perlop:

A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized.

I also believe that as the two ternary expressions are part of a (4-term) comma separated list, those four terms should be evaluated in strictly left to right order per perlop:

"In list context, it's just the list argument separator, and inserts both its arguments into the list. These arguments are also evaluated from left to right."

.

[quester]

Ah... yes. That would make it a bug. Sorry.

It doesn't seem to be the ternary operator causing the havoc, or pattern matching though:

$,=", ";
my $i;
print $i++, $i, $i++, $i, $i++, $i;

prints

0, 3, 1, 3, 2, 3

Using Dump, we can see that the optimizer is assuming that $1 must be constant for the length of time it takes to evaluate the list:

use Data::Dump::Streamer;
my $i;
print Dump \($i++, $i, $i++, $i, $i++, $i);

prints

$SCALAR1 = \do { my $v = 0 };
$SCALAR2 = \do { my $v = 3 };
$SCALAR3 = \do { my $v = 1 };
$SCALAR4 = $SCALAR2;
$SCALAR5 = \do { my $v = 2 };
$SCALAR6 = $SCALAR2;

The autoincrement is also not a necessary ingredient:

use Data::Dump::Streamer;
print Dump \("x"=~/(x)/, $1, "y"=~/(y)/, $1, "z"=~/(z)/, $1);

prints

$SCALAR1 = \do { my $v = 'x' };
$SCALAR2 = \do { my $v = 'z' };
$SCALAR3 = \do { my $v = 'y' };
$SCALAR4 = $SCALAR2;
$SCALAR5 = \do { my $v = 'z' };
$SCALAR6 = $SCALAR2;