How can I replace all hex using RegEx in PHP?

Question

So I've been toying around with Regular Expressions, and my friend challenged me to write a script that replaced all hex within a string. He gave me a large file mixed with different characters and, of course, some hex strings.

Each occurrence of hex is preceded with \x, so for example: \x55.

I thought it'd be pretty easy, so I tried out this pattern on some online regex tester: /\\x([a-fA-F0-9]{2})/

It worked perfectly.

However, when I throw it into some PHP code, it fails to replace it at all.

Can anyone give me a nudge into the right direction of where I'm going wrong?

Here's my code:

$toDecode = file_get_contents('hex.txt');
$pattern = "/\\x(\w{2})/";
$replacement = 'OK!';

$decoded = preg_replace($pattern, $replacement, $toDecode);

$fh = fopen('haha.txt', 'w');
fwrite($fh, $decoded);
fclose($fh);

Can you please provide a small snippet of your file hex.txt? — anubhava
– anubhava, Commented Apr 23, 2012 at 20:37
True hex only includes the characters 0-9 and A-F, while your regular expression will match characters other than these. — nickb
– nickb, Commented Apr 23, 2012 at 20:38
You could improve your recognition - \w is any letter or number, but hex is only 0-9 and A-F. You could replace it with [0-9a-fA-F] — Surreal Dreams
– Surreal Dreams, Commented Apr 23, 2012 at 20:42

Brad Christie · Accepted Answer · 2012-04-23 20:47:19Z

7

<?php
  // grab the encoded file
  $toDecode = file_get_contents('hex.txt');

  // create a method to convert \x?? to it's character facsimile
  function escapedHexToHex($escaped)
  {
    // return 'OK!'; // what you're doing now
    return chr(hexdec($escaped[1]));
  }

  // use preg_replace_callback and hand-off the hex code for re-translation
  $decoded = preg_replace_callback('/\\\\x([a-f0-9]{2})/i','escapedHexToHex', $toDecode);

  // save result(s) back to a file
  file_put_contents('haha.txt', $decoded);

For reference, preg_replace_callback. Also, don't use \w as it's actually translated to [a-zA-Z0-9_]. Hex is base-16, so you want [a-fA-F0-9] (and the i flag makes it case-insensitive).

Working example, minus the file part.

edited Apr 23, 2012 at 20:47

answered Apr 23, 2012 at 20:41

Brad Christie

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3 Comments

Rob Over a year ago

wouldn't preg_replace($pattern, chr(hexdec($1)), $toDecode) have the same effect?

Brad Christie Over a year ago

@Rob: Supposedly, though I had trouble getting it to work (even while using the e flag)

Rob Over a year ago

as did I. very strange. yours does indeed work, but now I want to know why my example above doesn't. Time to toy around with that. Thanks for the help!

DaveRandom · Accepted Answer · 2012-04-23 20:44:18Z

2

Your problem is that you have not escaped your backslashes in the PHP string. It needs to be:

$pattern = "/\\\\x(\\w{2})/";

~~...or:~~

~~$pattern = '/\\x(\w{2})/';~~

~~...with single quotes.~~ ^{- This actually suffers the same problem and requires the full double-escaped sequence}

But \w will match any perl word character, which is not just hex characters. I would use the character class [a-fA-F0-9] instead.

edited Apr 23, 2012 at 20:44

answered Apr 23, 2012 at 20:38

DaveRandom

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1 Comment

Rob Over a year ago

All this escaping gives me a headache, but that was indeed the problem. Thanks!

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