PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

Introducing Regular Expressions
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For example, the following matches bob at the start of a string:
^bob
This pattern matches com at the end of a string:
com$
Finally, this pattern matches a string containing only a single character from a to z:
^[a-z]$
Branching
You can represent a choice in a regular expression with a vertical pipe. For example, if
you want to match com, edu, or net, you can use the following expression:
com|edu|net
Matching Literal Special Characters
If you want to match one of the special characters mentioned in the preceding sections,
such as ., {, or $, you must put a backslash (\) in front of it. If you want to represent a
backslash, you must replace it with two backslashes (\\).
Be careful to put your regular expression patterns in single-quoted strings in PHP.
Using regular expressions in double-quoted PHP strings adds unnecessary complications.
PHP also uses the backslash to escape special characters—such as a backslash. If you want
to match a backslash in your pattern, you need to use two to indicate that it is a literal
backslash, not an escape code.
Similarly, if you want a literal backslash in a double-quoted PHP string, you need to
use two for the same reason.The somewhat confusing, cumulative result of these rules is
that a PHP string that represents a regular expression containing a literal backslash needs
four backslashes. The PHP interpreter will parse the four backslashes as two.Then the
regular expression interpreter will parse the two as one.
The dollar sign is also a special character in double-quoted PHP strings and regular
expressions.To get a literal $ matched in a pattern, you would need “\\\$”. Because this
string is in double quotation marks, PHP will parse it as \$, which the regular expression
interpreter can then match against a dollar sign.
Reviewing the Special Characters
A summary of all the special characters is shown in Tables 4.4 and 4.5.Table 4.4 shows
the meaning of special characters outside square brackets, and Table 4.5 shows their
meaning when used inside square brackets.