What does \x in PHP PCRE mean?

Question

From the manual:

After \x, up to two hexadecimal digits are read (letters can be in upper or lower case). In UTF-8 mode, \x{...} is allowed, where the contents of the braces is a string of hexadecimal digits. It is interpreted as a UTF-8 character whose code number is the given hexadecimal number. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8 character if the value is greater than 127.

So what does this mean?

The code point of "ä" is E4 while the UTF-8 representation is C3A4, but neiter of those matches:

$t = 'ä'; // same as "\xC3\xA4";

preg_match('/\\xC3A4/u', $t); // doesn't match
preg_match('/\\x00E4/u', $t); // doesn't match

With the curly braces it does match when I give the code point:

preg_match('/\\x{00E4}/u', $t); // matches

Answer 1

The syntax is a way to specify a character by value:

• \xAB specifies a code-point in the range 0-FF.
• \x{ABCD} specifies a code-point in the range 0-FFFF.

The indicated wording from the manual is bit confusing, perhaps in an attempt to be precise. Character values 128-255 (and some) are encoded as 2-bytes in UTF-8. Thus, a unicode regular expression will match 7-bit clean ASCII but will not match different encodings/codepages (i.e. CP437) that utilize values in said range. The manual is, in a roundabout way, saying that a unicode regular expression is only suitable to be used with correctly encoded input. However;

It doesn't mean that \xABCD is parsed as \x{ABCD} (one character). It is parsed as \xAB (one character) and then CD (two characters)¹. The braces address this parsing ambiguity issue:

After \x, up to two hexadecimal digits are read .. In UTF-8 mode, \x{...} is allowed ..

Some other languages use \u instead of \x for the longer form.

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¹ Consider that this matches:

preg_match('/\xC3A4/u', "\xC3" . "A4");