Can anyone tell me what is happening here?
<?php
// true
var_dump('\\ ' === '\ ');
// false
var_dump('\\\\ ' === '\\ ');
// true
var_dump('\\\\ ' === '\\\ ');
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2Have you tried printing the strings?JJJ– JJJ2012-03-28 12:58:55 +00:00Commented Mar 28, 2012 at 12:58
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1Yeah, dumped the hexcodes. They're really different, it seems that a \\ gets collapsed to \ for some reason, but it's in a single-quoted string, it shouldn't happen there.Dalibor Karlović– Dalibor Karlović2012-03-28 13:01:13 +00:00Commented Mar 28, 2012 at 13:01
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2There's no difference whether a string is single or double quoted. The escape character works identically in both cases.JJJ– JJJ2012-03-28 13:02:25 +00:00Commented Mar 28, 2012 at 13:02
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3 Answers
\ inside a string literal introduces several types of escape sequences, \\ is the escape sequence for a literal "\". But, \s that don't resolve to an escape sequence are also taken as literal "\".
Therefor, '\\ ' stands for the string "\ ", '\\\\ ' stands for the string "\\ ", just as '\\\ '. Try:
echo '\\\\ '; -> \\
See http://php.net/manual/en/language.types.string.php#language.types.string.syntax.single.
3 Comments
Dalibor Karlović
I had no idea that \\ works in a single-quoted string, thought it only works on ' itself.
DaveRandom
@DaliborKarlović It is set up like that so you can have a single quoted string that ends with a backslash, if you couldn't escape the backslash you wouldn't be able to wrap
string\ in single quotes.Dalibor Karlović
Yeah, I get it, my bad is that I was trying to use it inside a regex searching for a literal \n (not a control char), so preg_quote('#\n#') produced a string which matched, but var_dump()-ing the value produced by preg_quote() and using that instead did not. Now I had to use #(\\\n){2,}#
In single quoted strings, no escape sequences are interpolated. A backslash is only an escape character if it immediately precedes a single quote, or a backslash.
So:
var_dump('\\ '); // String (2) "\ "
var_dump('\ '); // String (2) "\ "
// They do match
var_dump('\\\\ '); // String (3) "\\ "
var_dump('\\ '); // String (2) "\ "
// They don't match
var_dump('\\\\ '); // String (3) "\\ "
var_dump('\\\ '); // String (3) "\\ "
// They do match
This is expected and documented behaviour, although it can be difficult to wrap you head around on the face of it.
Comments
In 1st example you're comparing
"\ " and "\ " which is TRUE
in 2nd
"\\ " and "\ " which is FALSE
in 3rd
"\\ " and "\\ " which is TRUE
If you print out your strings
$s = array('\ ', '\\ ', '\\\ ', '\\\\ ');
var_dump($s);
you'll get
array(4) {
[0]=>
string(2) "\ "
[1]=>
string(2) "\ "
[2]=>
string(3) "\\ "
[3]=>
string(3) "\\ "
}
All double-slashes '\\' have been converted into single-slashes '\' and sigle-slashes remain the same. Escaping works the same way inside single and double-quoted strings.
