Programming PHP

The Basics
Most characters in a regular expression are literal characters, meaning that they
match only themselves. For instance, if you search for the regular expression "cow" in
the string "Dave was a cowhand", you get a match because "cow" occurs in that string.
Some characters, though, have special meanings in regular expressions. For instance,
a caret (^) at the beginning of a regular expression indicates that it must match the
beginning of the string (or, more precisely, anchors the regular expression to the
beginning of the string):
ereg('^cow', 'Dave was a cowhand'); // returns false
ereg('^cow', 'cowabunga!'); // returns true
Similarly, a dollar sign ($) at the end of a regular expression means that it must
match the end of the string (i.e., anchors the regular expression to the end of the
string):
ereg('cow$', 'Dave was a cowhand'); // returns false
ereg('cow$', "Don't have a cow"); // returns true
A period (.) in a regular expression matches any single character:
ereg('c.t', 'cat'); // returns true
ereg('c.t', 'cut'); // returns true
ereg('c.t', 'c t'); // returns true
ereg('c.t', 'bat'); // returns false
ereg('c.t', 'ct'); // returns false
If you want to match one of these special characters (called a metacharacter), you
have to escape it with a backslash:
ereg('\$5\.00', 'Your bill is $5.00 exactly'); // returns true
ereg('$5.00', 'Your bill is $5.00 exactly'); // returns false
Regular expressions are case-sensitive by default, so the regular expression "cow"
doesn’t match the string "COW". If you want to perform a case-insensitive POSIX-style
match, you can use the eregi( ) function. With Perl-style regular expressions, you
still use preg_match( ), but specify a flag to indicate a case-insensitive match (as you’ll
see when we discuss Perl-style regular expressions in detail later in this chapter).
So far, we haven’t done anything we couldn’t have done with the string functions
we’ve already seen, like strstr( ). The real power of regular expressions comes
from their ability to specify abstract patterns that can match many different character
sequences. You can specify three basic types of abstract patterns in a regular
expression:
• A set of acceptable characters that can appear in the string (e.g., alphabetic characters,
numeric characters, specific punctuation characters)
• A set of alternatives for the string (e.g., "com", "edu", "net", or "org")
• A repeating sequence in the string (e.g., at least one but no more than five
numeric characters)
96 | Chapter 4: Strings
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