#!/bin/bash

echo
echo "String operations using \"expr \$string : \" construct"
echo "==================================================="
echo

a=1234zipper5FLIPPER43231

echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
#     Escaped parentheses grouping operator.            ==  ==

#       ***************************
#+          Escaped parentheses
#+           match a substring
#       ***************************


#  If no escaped parentheses ...
#+ then 'expr' converts the string operand to an integer.

echo "Length of \"$a\" is `expr "$a" : '.*'`."   # Length of string

echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."

# ------------------------------------------------------------------------- #

echo

echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`."
#                                                             ==      ==
echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`."
#         =====                                          ==       ==
# Again, escaped parentheses force a substring match.
#
echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`."
#         ====                  end of string operator  ^^
#  (In fact, means skip over one or more of any characters until specified
#+  substring found.)

echo

exit 0