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contains lots of theory on regular expressions as well. The Wikipedia page on regular expressions is great

for background information (

http://en.wikipedia.org/wiki/Regular_expression

).

A regular expression (sometimes referred to as a

regex

) is a special string that describes a text

pattern

.

With regular expressions you can define rules that match groups of strings, extract data from strings, and

transform strings, which enable very flexible and complex text manipulation using concise rules. Regular

expressions aren’t specific to mod_rewrite and URL rewriting. On the contrary, they’ve been around for a

while, and they’re implemented in many tools and programming languages, including PHP.

To demonstrate their usefulness with a simple example, assume your web site needs to rewrite links as

shown in Table 3-2.

Table 3-2

If you have 100,000 products, without regular expressions you’d be in a bit of a trouble, because you’d

need to write just as many rules — no more, no less.

You don’t want to manage an

.htaccess

file with

100,000 rewrite rules!

That would be unwieldy.

However, if you look at the Original URL column of the table, you’ll see that all entries follow the same

pattern

. And as suggested earlier, regular expressions can come to rescue! Patterns are useful because

with a single pattern you can match a theoretically infinite number of possible input URLs, so you just

need to write a rewriting rule for every

type

of URL you have in your web site.

In the exercise that follows, you use a regular expression that matches

Products/P

n

.html

, and use

mod_rewrite to translate URLs that match that pattern to

product.php?productID=

n

. This will

implement exactly the rules described in Table 3-1.

Working with Regular Expressions

1.

Open the

.htaccess

file you created earlier in your

seophp

folder, and add the following

rewriting rule to it:

RewriteEngine On

# Translate my-super.product.html to /product.php?product_id=123

RewriteRule ^my-super-product\.html$ /product.php?product_id=123

Original URL

Rewritten URL

Products/P1.html product.php?product_id=1

Products/P2.html product.php?product_id=2

Products/P3.html product.php?product_id=3

Products/P4.html product.php?product_id=4

…

…

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Chapter 3: Provocative SE-Friendly URLs

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