Top Forums Shell Programming and Scripting awk variables in regex expression ? Post 302486124 by michaelrozar17 on Friday 7th of January 2011 04:48:10 AM
Quote:
Originally Posted by vilius
Hello,

Could someone explain why this one returns nothing:
Code:
$ x=/jon/
$ echo jon | awk -v xa=$x '$1~xa {print}'
$

When a pattern matching is done using ~ we need to give the reg-exp in between / / (2 forward slashes).Still, when given inside the slashes it is taken as literally, so when changing the above awk as
Code:
echo jon | awk -v xa=$x '$1~/xa/ {print}'

it matches the word xa and not the variable xa's value.Hence we need to expand the variable as below
Code:
x=/jon/
echo jon | awk -v xa=$x '$1~/'"$xa"'/ {print}'

This is not the case when == is used as you would know this is used to equate between variables.Therefore in code $1==xa.. xa is expanded (to jon).
 
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English(3pm)						 Perl Programmers Reference Guide					      English(3pm)

NAME
English - use nice English (or awk) names for ugly punctuation variables SYNOPSIS
use English; use English qw( -no_match_vars ) ; # Avoids regex performance penalty # in perl 5.16 and earlier ... if ($ERRNO =~ /denied/) { ... } DESCRIPTION
This module provides aliases for the built-in variables whose names no one seems to like to read. Variables with side-effects which get triggered just by accessing them (like $0) will still be affected. For those variables that have an awk version, both long and short English alternatives are provided. For example, the $/ variable can be referred to either $RS or $INPUT_RECORD_SEPARATOR if you are using the English module. See perlvar for a complete list of these. PERFORMANCE
NOTE: This was fixed in perl 5.20. Mentioning these three variables no longer makes a speed difference. This section still applies if your code is to run on perl 5.18 or earlier. This module can provoke sizeable inefficiencies for regular expressions, due to unfortunate implementation details. If performance matters in your application and you don't need $PREMATCH, $MATCH, or $POSTMATCH, try doing use English qw( -no_match_vars ) ; . It is especially important to do this in modules to avoid penalizing all applications which use them. perl v5.18.2 2014-01-06 English(3pm)

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