PHP RegEx not matching a string that it should match

Question

This is driving me insane...

I have the following code:

    # open pdf
    $pdf = file_get_contents('myfile.pdf');

    echo("RE 1:\n");
    preg_match('/^[0-9]+ 0 obj.*\/Contents \[ ([0-9]+ [0-9]+) R \\]/msU', $pdf, $m);
    var_dump($m);

    echo("\nRE 2:\n");
    preg_match('/^8 0 obj.*\/Contents \[ ([0-9]+ [0-9]+) R \\]/msU', $pdf, $m);
    var_dump($m);

The file myfile.pdf contains the following text:

...
8 0 obj
<<
/Type /Page
/Parent 2 0 R
/Resources 6 0 R
/Contents [ 5 0 R ]
>>
endobj
...

The only difference between those two regular expressions is the numeric range at the beginning of the string. Yet I get the following output:

RE 1:
array(0) {
}

RE 2:
array(2) {
  [0]=>
  string(78) "8 0 obj
<<
/Type /Page
/Parent 2 0 R
/Resources 6 0 R
/Contents [ 5 0 R ]"
  [1]=>
  string(3) "5 0"
}

I would expect both regular expressions to return similar results, but the regular expression with the numeric range at the start (RE 1) doesn't return any results. Is this a bug or am I doing something wrong?

Update

After adding preg_last_error(), I am getting PREG_BACKTRACK_LIMIT_ERROR. How can I fix that?

@Emma Yes, that is what I'm trying to capture. It works perfectly on regex101.com, but not in my code. — Ryan Steffer
– Ryan Steffer, Commented Jul 29, 2019 at 18:39
Both of your regexes work fine at sandbox.onlinephpfunctions.com so it could be that your PHP or PCRE version is causing a headache? — MonkeyZeus
– MonkeyZeus, Commented Jul 29, 2019 at 18:42
Try using preg_last_error() to see if it gives you any hints. — MonkeyZeus
– MonkeyZeus, Commented Jul 29, 2019 at 18:44
@MonkeyZeus Good call! I am getting PREG_BACKTRACK_LIMIT_ERROR. — Ryan Steffer
– Ryan Steffer, Commented Jul 29, 2019 at 18:52
Check your php.ini file and see what pcre.backtrack_limit is set to or use echo ini_get( 'pcre.backtrack_limit' ); if you don't have access to php.ini — MonkeyZeus
– MonkeyZeus, Commented Jul 29, 2019 at 18:54

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

I'm guessing that you might be designing an expression that would somewhat look like,

[0-9]+\s+0\s+obj\b.*?\/Contents\s+\[\s*([0-9]+\s+[0-9]+)\s+R\s*\]

on s mode.

Test

$re = '/[0-9]+\s+0\s+obj\b.*?\/Contents\s+\[\s*([0-9]+\s+[0-9]+)\s+R\s*\]/s';
$str = '8 0 obj
<<
/Type /Page
/Parent 2 0 R
/Resources 6 0 R
/Contents [ 5 0 R ]
>>
endobj

8 0 obj
<<
/Type /Page
/Parent 2 0 R
/Resources 6 0 R
/Contents [ 5 0 R ]
>>
endobj';

preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);

var_dump($matches);

The expression is explained on the top right panel of regex101.com, if you wish to explore/simplify/modify it, and in this link, you can watch how it would match against some sample inputs, if you like.

RegEx Circuit

jex.im visualizes regular expressions:

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 29, 2019 at 18:40

Emma Marcier

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6 Comments

MonkeyZeus Over a year ago

OP is using /msU so their . matches everything including newlines.

Ryan Steffer Over a year ago

Yes, but that RE does work. And my output of preg_last_error() is PREG_BACKTRACK_LIMIT_ERROR. So that's why mine doesn't work I guess. But I'm not sure what causes that...

Ryan Steffer Over a year ago

Yours works with /msU. I'm wondering if it's the word boundary you used... Testing more things now.

Ryan Steffer Over a year ago

Ok, yours works because you added the .*? quantifier. I am already using the /U modifier, which means Ungreedy. But then your .*? reverses that. But I need it to be ungreedy, as there are many instances of this string I'm trying to capture.

Ryan Steffer Over a year ago

I see that. It will fail in regex101.com. But both are acceptable in PHP. I have tried both and they yield the exact same result.

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