{QUERY STRING} in URL write

[JohnsonHk]

I found a URL REwrite as follows:
index.php?main=index&c=202

RewriteRule ^(.*)_c([0-9]+)$ index\.php?main=index&c=$2&%{QUERY_STRING} [L]

but I am not sure whether it is right or not, and what is the usage of {QUERY_STRING} in this rule?

[g1smd]

It re-appends the original query string data.

The (.*) part of the pattern is dangerous in several ways:
- it isn't passed as $1 to your script and therefore its value is not checked or validated: that's a duplicate content issue.
- the usage of (.*) as a pattern is very inefficient, requiring multiple back-off and retry attempts before a match can be found. If the following underscore is the very first underscore in the URL change the pattern to "match 'not an underscore' until the first underscore", like:

([^_]+_)

or similar.

Do not escape the period in the target filepath. Only escape them in patterns.