Using sed and regex for text case conversion

Replacing lowercase hexcodes with uppercase.

sed -E 's/#[[:alnum:]]+/\U&/g' foobar.xml

1. # hashtag
2. [:alnum:] is a POSIX bracket expression and captures all Alphanumeric characters [a-zA-Z0-9]
3. + quantifier 1 to many
4. \U& match is converted to uppercase - more info

GNU sed uses POSIX.2 BREs. The -E option switches to using extended regular expressions instead.