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Using Table 3-2 as reference, analyze the expression

^Products/P([0-9]*)\.html$

. The expression

starts with the

^

character, matching the beginning of the requested URL (remember, this doesn’t include

the domain name). The characters

Products/P

assert that the next characters in the URL string match

those characters.

Let’s recap: the expression

^Products/P

will match any URL that starts with

Products/P

.

The next characters,

([0-9]*)

, are the crux of this process. The

[0-9]

bit matches any character between

0 and 9 (that is, any digit), and the

*

that follows indicates that the pattern can repeat, so you can have an

entire number rather than just a digit. The enclosing parentheses around

[0-9]*

indicate that the regular

expression engine should store the matching string (which will be a number) inside a variable called

$1

.

(You’ll need this variable to compose the rewritten URL.)

Finally, you have

\.html$

, which means that string should end in

.html

. The

\

is the escaping charac-

ter, which indicates that the

.

should be taken as a literal dot, not as “any character” (which is the signif-

icance of the

.

metacharacter). The

$

matches the end of the string.

The second argument of

RewriteRule

,

/product.php?product_id=$1

, plugs the variables that you

stored into your script URL mapping, and indicates to the web server that requests by a browser for

URLs matching that previous pattern should be delegated to that particular script with those numbers

substituted for

$1

.

Metacharacter Description

( )

The parentheses are used to define a

captured expression

. The string matching the

expression between parentheses can then be read as a variable. The parentheses

can also be used to group the contents therein, as in mathematics, and operators

such as

*

,

+

, or

?

can then be applied to the resulting expression.

[ ]

Used to define a character class. For example,

[abc]

will match any of the charac-

ters

a

,

b

,

c

. The

-

character can be used to define a range of characters. For example,

[a-z]

matches any lowercase letter. If

-

is meant to be interpreted literally, it should

be the last character before

]

. Many metacharacters lose their special function when

enclosed between

[

and

]

, and are interpreted literally.

[^ ]

Similar to

[ ]

, except it matches everything except the mentioned character class.

For example,

[^a-c]

matches all characters except

a

,

b

, and

c

.

$

Matches the end of the line. In our case, it will always match the end of the URL.

It is useful to think of it as “anchoring” the previous characters to the end of the

string, that is, asserting that they be the last part.

\

The backslash is used to escape the character that follows. It is used to escape

metacharacters when you need them to be taken for their literal value, rather

than their special meaning. For example,

\.

will match a dot, rather than “any

character” (the typical meaning of the dot in a regular expression). The backslash

can also escape itself — so if you want to match

C:\Windows

, you’ll need to refer

to it as

C:\\Windows

.

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Chapter 3: Provocative SE-Friendly URLs

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