get number of latin letter in alphabet simple

#!/bin/bash

#get number of latin letter in alphabet
#use nlett <letter>, e.g nlett a

SNUM=0
SFND=`echo $1|grep '^[a-z]$'`
if [ -z "$SFND" ]; then
    echo "Error: in first parameter must be one small latin letter!"
    exit 1
fi

ALPHABET="abcdefghijklmnopqrstuvwxyz"
SNUM=`expr index $ALPHABET $SFND`

echo "$SFND is $SNUM"