How to escape backslashes to have safe strings ? php

Question

i am stuck in this question since yesterday, i just want to understanding how to escape strings correctly, i didn't found any full answer.

here is my issue =>

i have tow files:

1. replacement.txt , here is the content:

   new_content '\0020'

2. subject.txt , here is the content:

   structure old_content

here is my code:

$myfile = fopen ( __DIR__ . '/replacement.txt', "r" ) or die ( "Unable to open file!" );
$replacement = fread ( $myfile, filesize ( __DIR__ . '/replacement.txt' ) );
fclose ( $myfile );


    $myfile = fopen ( __DIR__ . '/subject.txt', "r" ) or die ( "Unable to open file!" );
    $subject = fread ( $myfile, filesize ( __DIR__ . '/subject.txt' ) );
    fclose ( $myfile );

    $subject = preg_replace ( "%structure.*%s", $replacement, $subject, - 1, $count );
    die ( $subject );

this code should print : new_content '\0020' but it prints new_content 'structure old_content20' .why? because there is a backslash that not escaped.

here is what i did but there is no luck :(

• i had tried to add $replacement = addslashes($replacement); before preg_replace: but here is what i got:new_content \'\0020\', incorrect because there is a backslashed before quotes.

• i had tried strip slash the previous result $subject = stripslashes ( $subject ), but here is what i got: new_content '020' , incorrect !

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the only thing that worked for me, is to edit the replacement.txt to this : new_content '\\0020'. but i can't change that manually , because it's an input from the user.

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i don't think that the problem with the backslash , because if i change the replacement.txt to new_content '\anyAlaphbetCharacter' the above code (without addslahes and stripslash) will print : new_content '\anyAlaphbetCharacter' and that's correct.

please help me to understand what's going on ?and what the solution ?

Answer 1

i would suggest you one trick that is to replace \ with some special word like [backslash] before preg_replace to avoid any problem with it

$replacement = str_repalce(\`, [backslash], $replacement);`

then replace it back after preg_replace call

$subject = str_repalce([backslash],\`,$subject);`

Answer 2

What is going on. I think php defines \0 simbol as "\0" (ASCII 0 (0x00)), the NUL-byte.

Try use preg_quote() function.