PHP json Scanning escaped character - php

Question

I am writing a JSONScanner class that basically takes a string and scans the whole thing to construct a JSONObject. Currently I'm writing read_string() method, to read a string. When reading a string that escapes '\', I get some invalid output.
Here is my JSONScanner class
class JSONScanner {
private $in;
private $pos;
public function __construct($in) {
$this->in = $in;
$this->pos = 0;
}
#########################################################
############### Method used for debugging ###############
#########################################################
public function display() {
$this->pos = 1;
echo $this->read_string($this->get_char());
}
#########################################################
#########################################################
private function read_string($quote) {
$str = "";
while(($c = $this->get_char()) != $quote) {
if($c == '\\') {
$str .= $this->get_escaped_char();
} else {
$str .= $c;
}
}
return $str;
}
private function get_escaped_char() {
$c = $this->get_char();
switch($c) {
case 'n':
return '\n';
case 't':
return '\t';
case 'r':
return '\r';
// display the characters being escaped
case '\\':
case '\'':
case '"':
default:
return $c;
}
}
private function get_char() {
if($this->pos >= strlen($this->in)) {
return -1; // END OF INPUT
}
return substr($this->in, $this->pos++, 1);
}
}
Here is my running code
$str = '{"a\\":1,"b":2}';
$jscan = new JSONScanner($str);
$jscan->display();
With the above string, I'm getting
a":1,
However when I try
$str = '{"a\\\":1,"b":2}';
$jscan = new JSONScanner($str);
$jscan->display();
I get what I need, which is
a\
Why am I needing to put 2 backslashes to escape 1 backslash?
EDIT:
I was trying the same json string on json_decode, and it gave me the same results, with 2 backslashes, nothing but with 3 backslahes it gave me a\. Why is that? Isn't escaping a backslash takes 2 consecutive ones \\?

Answer 1

$str = '{"a\\":1,"b":2}';
This is a PHP string literal, which has its own escaping rules. The actual string you're representing with the above is:
{"a\":1,"b":2}
If you want to represent one backslash in a PHP string literal, you need to write two backslashes. So the correct string representation for what you want is:
$str = '{"a\\\\":1,"b":2}';
It happens to work with three backslashes, because \\ becomes one \ and the next \ isn't followed by any special character, so it by itself also represents a single backslash.