How to converting underscore to hyphens

[blu32008]

this is my code to rewrite my url for seo friendly

RewriteRule products-([a-z]+)-([0-9]+)-(.*)-([a-z]+)\.html products.php?on=$1&pid=$2&pt=$3&view=$4 [L]
RewriteRule products-([a-z]+)-([0-9]+)-(.*)\.html products.php?on=$1&pid=$2&pt=$3 [L]

when the script is execute, the result looks like this:

products-details-400-patriot_xporter_xt_boost_32gb_usb_flash_drive_180x_speed.html

the bold text is my product title -> $3.
How can i convert the underscore on $3 into hypens. Thx before

[richardk]

What are the possible values of view? If you can't put them in the first rule you will have a problem working out if the last part is for view or pt.

[blu32008]

a-z only...so the view cannot be an empty values ?

[richardk]

Yes. The problem is the optional view. One way would be to always have a view. Another would be to always put a - after pt even if there is no view.