expr.sh

#!/bin/bash

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

a=1234zipper5FLIPPER43231

echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
#     转义括号对操作.                                   ==  ==

#       ***************************
#+              转移括号对
#+           用来匹配一个子串
#       ***************************


#  如果不转义括号的话...
#+ 那么 'expr' 将把string操作转换为一个整数.

echo "Length of \"$a\" is `expr "$a" : '.*'`."   # 字符串长度

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" : '\(.......\)'`."
#         =====                                          ==       ==
# 再来一个, 转义括号对强制一个子串匹配.
#
echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`."
#         ====                  end of string operator  ^^
#  (最后这个模式的意思是忽略前边的任何字符,直到最后7个字符,
#+  最后7个点就是需要匹配的任意7个字符的字串)

echo

exit 0