Another useful command could be
[rocketmouse at archlinux ~]$ man rename | grep EXAMPLES -A10
EXAMPLES
Given the files foo1, ..., foo9, foo10, ..., foo278, the commands
rename foo foo0 foo?
rename foo foo0 foo??
will turn them into foo001, ..., foo009, foo010, ..., foo278.
And
rename .htm .html *.htm
will fix the extension of your html files.