The regular expression standard that made its way through the POSIX standard is perhaps the simplest form of regex available to PHP programmers. As such, it makes a great learning tool because the functions that implement it do not provide any particular "advanced" features.

In addition to the standard rules that we have already discussed, the POSIX regex standard defines the concept of character classes as a way to make it even easier to specify character ranges. Character classes are always enclosed in a set of colon characters (:) and must be enclosed in square brackets. There are 12 character classes:

• alpha represents a letter of the alphabet (either upper- or lowercase). This is equivalent to [A-Za-z].

• digit represents a digit between 09 (equivalent to [0-9]).

• alnum represents an alphanumeric character, just like [0-9A-Za-z].

• blank represents "blank" characters, normally space and Tab.

• cntrl represents "control" characters, such as DEL, INS, and so forth.

• graph represents all the printable characters except the space.

• lower represents lowercase letters of the alphabet only.

• upper represents uppercase letters of the alphabet only.

• print represents all printable characters.

• punct represents punctuation characters such as "." or ",".

• space is the whitespace.

• xdigit represents hexadecimal digits.

This makes it possible, for example, to rewrite our email validation regex as follows:

[[:alnum:]_]+@[[:alnum:]_]+\.[[:alnum:]_]{2,4}
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