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Firstly, apologies if this is not the correct type of question for here, I had it on the stackoverflow but it was closed with a suggestion I post here.

I’m in the process of converting from Latin 15 to Unicode/UTF-8 and researched several tutorials, and am looking here for a critique of what I have implemented based on them (or, IOW, did I understand it!) :

I want to parse form data in PHP to ensure it is safe from SQL injection and email header attacks, and any other security holes I've not considered.

Although I’m using UTF-8, I just want to cater for English plus some of the extra acute, tilde etc. characters that would normally encounter, plus the Euro symbol. Everything else is disallowed and throws an error, as opposed to silently being replaced/removed.

This is my code so far:

// ensure it's valid unicode / get rid of invalid UTF8 chars
$text = iconv("UTF-8","UTF-8//IGNORE",$text);

// and just allow a basic english...ish.. chars through - no controls, chinese etc
$match_list = "\x{09}\x{0a}\x{0d}\x{20}-\x{7e}"; // basic ascii chars plus CR,LF and TAB 
$match_list .= "\x{a1}-\x{ff}"; // extended latin 1 chars excluding control chars
$match_list .= "\x{20ac}"; // euro symbol

if (preg_match("/[^$match_list]/u", $text) )
    $error_text_array[] = "<b>INVALID UNICODE characters</b>";

This code should only allow the characters shown in yellow here http://solomon.ie/unicode/

My main question is does this do as I want (it seems to work) or have I missed anything? My understanding of this is that the iconv function will remove any invalid sequences (i.e. hack attempts at the bit level) and leave just valid UTF-8, then my regexp checks for the characters I want allowed.

Although it seems to work I’m still confused by the regexp and the hex notation. Am I matching the Unicode code points, or, the actual binary UTF-8 representation of those code points? * It appears to be the former.

*my understanding is that a codepoint is basically the virtual location of a specific character in the Unicode “world” or characters, i.e. Euro symbol is the 20ACth character from the start, but it’s actual binary code, and number of bytes, is depended on if you use UTF-8, or 16 or 2 etc. So the codepoint never changes but the bit sequence can.

And, I have seen both, say, \x{20}-\x{7e} and \x20-\x7e used – what is the difference with and without the braces?

I intend to use the above few lines on all form fields, then follow it by further checks depending on the nature of the input.

For example if expecting an integer of the 0-9 kind, can I just use preg_match("/[^0-9]/", $text) without the /u modifier and specify literal characters (from x00 to x7e)? And suppose I want to allow 0-9 and the Euro in, is this the correct way preg_match("/[^0-9\x{20ac}]/u", $text)? And if I’m expecting a hidden field with ”ADD” or “EDIT” is if (!preg_match("/^(ADD|EDIT)$/", $text)) is still valid to test that?

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I don't see why they closed it and told you to move it here. This is a legitimate question and not so much a code review. At least if I am understanding your issue correctly, you are just looking for clarification. Someone here might be able to help, but all I know about regex is that some alien wrote it, so good luck! –  mseancole Apr 27 '12 at 13:05
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1 Answer

Your code seems legit to me, so I'll try to answer some of your questions. preg_match() uses UTF-8 input based on code points, so you don't have to worry about that. The difference between \xYY and \x{YYYY} is that the first one accepts either one or two characters after control \x (so, up to 256-th code point), and the latter one is universal and accepts range which covers all Unicode table. The u flag for RegExp pattern indicates that engine should treat the pattern as Unicode, it doesn't do anything with the text under test. So you can omit this flag in simple patterns like /[0-9]/. The last two code snippets also correct.

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