Shell Programming and Scripting

var='Happy_world_foo123
Happy me 2
Nobody like my foo6564'

echo "${var#*foo}"
123
Happy me 2
Nobody like my foo6564

echo ${var#*foo}
123 Happy me 2 Nobody like my foo6564

echo using sed:
sed -n '/foo/s/[^0-9]*\([0-9]*\).*/\1/p' in
echo using ask:
awk '/foo/{if(match($0, /[0-9]+/)) printf("%s\n", substr($0, RSTART, RLENGTH))}' in

$ echo Happy_world_foo123 | sed '/foo/s/[^0-9]//g'
123

$ echo Happy_world_foo123 | sed '/foo/s/[^0-9]//g'

var='Happy_world_foo123
Happy me 2
Nobody like my foo6564'

echo "$var" | sed '/foo/s/[^0-9]//g'
123
Happy me 2
6564

$ echo "$var" | nawk '{gsub("[A-Za-z_]","",$0)}1'
123
2
6564