For non-Unicode setups this means character in the range a-z, numbers and _, for Unicode
setups it means all characters except space, &, ;, /, :, =, ?, [, ], (, ), -.
Invalid characters are converted to -.
Example with a non-Unicode setup
'My car' => 'My-car'
'What is this?' => 'What-is-this'
'This & that' => 'This-that'
'myfile.tpl' => 'Myfile-tpl',
'øæå' => 'oeaeaa'