Question

Within a regex seems that it is the same:

if(preg_match('/\x{61}/',"a",$m)) //Same as just \x61
    print_r($m);

But if you make echo both apart.

echo "\x61";//Display a
echo "\x{61}";//Display \x{61}

Note: Not confuse with the '/\x{0000}/u'

I don’t see info about that and I’m not sure which to use.

Answer 1

pcre has its own escape rules, which are different from those in php. From http://www.pcre.org/pcre.txt

Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x. There is no difference in the way they are han-
dled. For example, \xdc is exactly the same as \x{dc}.