Advanced Bash−Scripting Guide

echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0−9]*\)'`
# Note that escaped parentheses
# == == + trigger substring match.
echo "The digits at the beginning of \"$a\" are \"$b\"."
echo
exit 0
Advanced Bash−Scripting Guide
The : operator can substitute for match. For example, b=`expr $a : [0−9]*` is the
exact equivalent of b=`expr match $a [0−9]*` in the above listing.
#!/bin/bash
echo
echo "String operations using \"expr \$string : \" construct"
echo "==================================================="
echo
a=1234zipper5FLIPPER43231
echo "The string being operated upon is \"`expr "$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 ^^
# (actually means skip over one or more of any characters until specified
#+ substring)
echo
exit 0
This example illustrates how expr uses the escaped parentheses −− \( ... \) −− grouping operator in tandem
with regular expression parsing to match a substring.
Chapter 12. External Filters, Programs and Commands 157