Parsing the following with awk:

$> df -h  /
Filesystem      Size  Used Avail Use% Mounted on
rootfs          476G  370G  106G  78% /

If I use an explicit match for the G’s on the values, it works as expected:

$> awk -v indrive="/dev/sda1" 'NR!=1{gsub(/G/,""); print $2,$4,indrive}' <(df -h /)
476 106 /dev/sda1

However, if I genericize it w/a char class:

awk -v indrive="/dev/sda1" 'NR!=1{gsub(/[[:alpha:]]/,""); print $2,$4,indrive}' <(df -h /)
370 78% /dev/sda1

Not sure where 370 and 78% are coming from.

Update: I actually get the same from:

awk -v indrive="/dev/sda1" 'NR!=1{gsub(/[a-zA-Z]/,""); print $2,$4,indrive}' <(df -h /)
370 78% /dev/sda1

But with [[:upper:]] it seems to work fine:

awk -v indrive="/dev/sda1" 'NR!=1{gsub("([[:upper:]])*",""); print $2,$4,indrive}' <(df -h /)
476 106 /dev/sda1