Position of a string within a string using Linux shell script?

With bash

a="The cat sat on the mat"
b=cat
strindex() { 
  x="${1%%"$2"*}"
  [[ "$x" = "$1" ]] && echo -1 || echo "${#x}"
}
strindex "$a" "$b"   # prints 4
strindex "$a" foo    # prints -1
strindex "$a" "ca*"  # prints -1