Programming PHP

POSIX-Style Regular Expressions

Now that you understand the basics of regular expressions, we can explore the details. POSIX-style regular expressions use the Unix locale system. The locale system provides functions for sorting and identifying characters that let you intelligently work with text from languages other than English. In particular, what constitutes a “letter” varies from language to language (think of à and ç), and there are character classes in POSIX regular expressions that take this into account.

However, POSIX regular expressions are designed for use with only textual data. If your data has a NUL-byte (\x00) in it, the regular expression functions will interpret it as the end of the string, and matching will not take place beyond that point. To do matches against arbitrary binary data, you’ll need to use Perl-compatible regular expressions, which are discussed later in this chapter. Also, as we already mentioned, the Perl-style regular expression functions are often faster than the equivalent POSIX-style ones.

Character Classes

As shown in Table 4-7, POSIX defines a number of named sets of characters that you can use in character classes. The expansions given in Table 4-7 are for English. The actual letters vary from locale to locale.

Table 4-7. POSIX character classes

Class	Description	Expansion
[:alnum:]	Alphanumeric characters	[0-9a-zA-Z]
[:alpha:]	Alphabetic characters (letters)	[a-zA-Z]
[:ascii:]	7-bit ASCII	[\x01-\x7F]
[:blank:]	Horizontal ...