How to uppercase file name but exclude extension

Is it possible to use rename to uppercase a file but exclude its extension?

ie: I want to rename the file foo_bar.ext to FOO_BAR.ext

I tried with rename 'y/a-z/A-Z/' foo_bar.ext, but the whole file (including extension) gets uppercased FOO_BAR.EXT

Answer

You are asking rename to convert all the instances of [a-z] to [A-Z]. Instead, capture the desired string into a group and modify it:

rename 's/([^.]*)/U$1/' foo_bar.ext

This would rename the file foo_bar.ext to FOO_BAR.ext.

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If you have a file foo_bar.baz.ext that needs to be renamed to FOO_BAR.BAZ.ext, use greedy match and multiple groups. Saying:

rename 's/(.*)(..*)/U$1E$2/' foo_bar.baz.ext

would rename the file foo_bar.baz.ext to FOO_BAR.BAZ.ext.