ExpressionEngine RewriteRule RegEx Throws 500 Error - regex

Question

When using categories in ExpressionEngine, a Category URL Indicator trigger word can be set to load a category by its {category_url_title}.
I would like to remove the category "trigger word" from the URL.
Here is what I have so far, with the trigger word set to "category":
RewriteRule /products/(.+)$ /products/category/$1 [QSA,L]
I'm not an expert at writing regular expressions, but I do a little. I'm 99% sure my RegEx is fine, however when trying to use it as a RewriteRule in my .htaccess file, I'm getting a 500 error.
I'm sure it's something stupid, but for some reason I'm not seeing my mistake. What am I doing wrong?
Update: Adding a ^ to the beginning of the RewriteRule fixed the 500 error.
RewriteRule ^/products/(.+)$ /products/category/$1 [QSA,L]

Answer 1

This is not safe. Take:
/products/a
The regex group matches a.
It will be rewritten to:
/products/category/a
which the regex matches again (this time, the group matches category/a). Guess what will happen.
You want /products/ from the beginning of input if it is not followed by category/, which means you want a negative lookahead. Also, the QSA flag is of no use, you don't have a query string to rewrite (QSA stands for Query String Append):
RewriteRule ^/products/(?!category/)(.+) /products/category/$1 [L]
Another way to use it (and which I personally prefer) is to use a RewriteCond prior to the rule:
RewriteCond %{REQUEST_URI} ^/products/(?!category/)
RewriteRule ^/products/(.*) /products/category/$1 [L]

Answer 2

This Apache RewriteRule should do the job for you*:
RewriteCond %{REQUEST_URI} ^/products/(?!category/)
RewriteRule ^/products/(.*) /products/category/$1 [L]
With this in place, you'll need to hard code your category links manually:
{categories backspace="2"}
{category_name},
{/categories}
Which would output the new Category URLs you desire:
http://example.com/products/toys
Otherwise, if using the recommended path variable when building your category links:
{categories backspace="2"}
{category_name},
{/categories}
Would create links with the Category URL Indicator in the URI:
http://example.com/products/C1
http://example.com/products/category/toys
Which — while perfectly valid — would create canonicalization issues on your site since the different URLs would appear as duplicate content to search engines.
*Credit to fge for brilliant mod_write rule.