Writing .htaccess RewriteRules

RewriteRules

Rewriterules are the heart and sole of the mod rewrite, here is where you declare the file to be rewritten, where it is to be rewritten and tack on any special commands.

Rewrite rules are broken down into 4 simple blocks. I'll refer to these blocks as the Call to action, Pattern, Rewrite and Command Flag.

Example:
RewriteRule ^dir/([0-9]+)/?$ /index.php?id=$1 [L]

Call to action: RewriteRule
Pattern: ^dir/([0-9]+) /?$
Rewrite: /index.php?id=$1
Command Flag: [L]

Between each of these blocks of the rewrite rule there should be a space. With that being said let's go ahead and break down each of these 4 blocks and discuss what they do.


Call to action Block

The only way to screw this up is to spell RewriteRule incorrectly or leave out the space between this and the starting of the pattern block. If you do spell it incorrectly you'll trigger an error and the browser will out put a 500 error. Note if you ever see a 500 error on your site it mostlikely due to a bad line of code in your .htaccess file.

Pattern Block

This one little piece of the mod rewrite is where the power is. In the pattern block of the rewrite rule we use regular expressions to detect the requested file name or uri and from this we can extract key parts to pass to the rewrite block.

Pay attention because this is the hardest part of mod rewrite.


Regular expressions is just a method to detect letters, numbers and symbols using special characters. These special characters are called metacharacters.

Pattern Matching metacharacter Definitions

Char.	Definition
\	Use before any of the following characters to escape or null the meaning or it. \* \. \$ \+ \[ \]
^	Start matching at this point
$	End point of the match
.	Any character
[]	Starts a class
|	Starts alternative match this|that would mean match this or that
()	starts a back reference point
?	match 0 or 1 time Quantifier
+	match atleast 1 or more times Quantifier
*	match 0 to infinite times Quantifier
{}	match minimum to maximum Quantifier {0,3} match up to 3 times

Class Definitions []

Char.	Definition
^	Negates the class. [^A-Z]+ means don't match any uppercases
\	Use before any of the following characters to escape or null the meaning or it. [\+]+
-	Range for matching [0-9]+ [a-zA-Z]+

I'll show a few quick samples just so you understand how to use all of the above. Then we're going to move right on to the Rewrite Block since we'll be going over all of this in our basic section.

In this example we just need the numbers in the ulrs below to pass through the mod rewrite to make our query. First we have to ask ourselves, "What is the common pattern in these urls"?

In this example there are two common patterns that we can match against. The first one is they all start with category/. The second is they all end in .htm. This should be an easy match

1. category/1.htm

1. category/56.htm

1. category/092340923.htm

1. category/9334.htm

So to use regular expressions to match all of these urls below we need to set our starting point to ^category/.

Now we need to tell the rewrite rule to look for any number 1 or more times. We'll use a character class to do this [0-9]+. Since we need this number to complete our rewrite block we're going to tell the mod to reference this so we can use it later. We do this by surrounding the the
[0-9]+ with brachets like this ([0-9]+).

To finish the match we're going to negate the . (remember this means any 1 character) even though a . is considered 1 character we're going to go ahead and negate it to read as a dot and then finish the match with htm$.

Mouse over the characters for a definition:

RewriteRule ^category/([0-9]+)\.htm$ /category.php?cat_id=$1 [L]


Another Regular Expression I recently wrote for matching letters,numbers,spaces,underscores is as follows

/(^[A-Za-z][\w\s]*)$/

Source: http://www.webforgers.net/mod-rewrite/mod-rewrite-syntax.php#quantifiers