php's perl regexp engine issue.

spaceDAISY

It seems to me the more i use php's so-called perl regular expression engine, the more it seems that it's actually nothing like perl's regular expression engine at all. Take these two insanely simple pieces of code:

$t = "goiter";

if (preg_match("/\$/", $t)) {
        print "Hi!";
}

This prints "Hi!", which i would consider to be very incorrect behaviour, as there most certainly isn't a $ in the string "goiter". Take this perl code:

$t = "goiter";

if ($t =~ /\$/) {
print "Hi!";
}

almost identical, with precisely the same regexp - and it does the right thing, it doesn't print "Hi!".

Does anyone know what's going on here? I've been battling against really really simple issues like this ever since i've picked up php, and to be honest, it frustrates the hell out of me. This is the second simplest regexp you can possibly do, short of matching a single non-escaped charater, and yet it doesn't appear to work in the way it should. I'm clearly doing something wrong, yet I can't for the life of me see what it could be. Any ideas? Are there any resources for perl programmers making the move into php that point out these sorts of pitfalls?

laserlight

Did you try:

<?php
$t = "goiter";

if (preg_match("/\\$/", $t) == 1) {
	print "Hi!";
}
?>

btw, dont use the [php] bbcode tags if you are going to post characters escaped with backslashes, the parser parses them out anyway.

EDIT:
conversely, you could use:

<?php
$t = "goiter";

if (preg_match('/\$/, $t) == 1) {
	print "Hi!";
}
?>

The issue is more of a double quote vs single quote thing.
Basically, PHP considers "\$" to be an escaped $ sign.
This means that within a pattern, it is treated as a $ sign, not an escaped $ sign.
But the $ sign is the end marker, so the actual content of the pattern is blank.
Since the null set is a subset of all sets, there is a match, so "Hi!" is printed.

However, the $ sign does not need to be escaped within single quotes, as such, \$ is \$, so the output is as expected.

spaceDAISY

Ok, yes that makes sense, in perl you use " " to interpolate, and ' ' to not interpolate the content, so it seems reasonable that it works like this in php too. I guess i was confused as every single example of php code i've seen thus far has used " ", i guess with the expectation that they might want to interpret a variable at some stage.

Yet another example of me not looking around enough and getting frustrated before i'm supposed to, i guess 🙂