.* means "go as far as you can using any character" so of course that matches the whole string (and \S matches the last non space character). If you only want non space chars after 0x you can use: 0x\S* instead .

Thanks Eily, this was the most simplest solution

Another statement of the problem might be "after 'addr:0x', replace 1 or more hex digits (or maybe just a single '0'?) with a string from a variable". The s/// match pattern might be expressed as
    (?<= addr:0x)  # after a certain literal string
    [[:xdigit:]]+  # 1 or more hex digits
(Update: See "(?<=pattern)" (and friends) in Extended Patterns, "Lookaround Assertions" subsection, in perlre; Looking ahead and looking behind in perlretut. See also POSIX Character Classes in perlrecharclass for [[:xdigit:]] and similar.)

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = qq{0 (0) tracegen chn:req cmd:SDP_CMD_RDSIZED unit:0x10 addr:
+0x0 len:0x3f vc:0 qospri:0 qosfw:0 strm:0x0 chain:0 io:0\n};
 ;;
 my $address = '500000000';
 ;;
 $s =~ s{ (?<= addr:0x) [[:xdigit:]]+ }{$address}xms;
 ;;
 my $expected = qq{0 (0) tracegen chn:req cmd:SDP_CMD_RDSIZED unit:0x1
+0 addr:0x500000000 len:0x3f vc:0 qospri:0 qosfw:0 strm:0x0 chain:0 io
+:0\n};
 print 's/// ok' if $s eq $expected;
"
s/// ok
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If you have Perl version 5.10+, you can use the more flexible  \K (see Extended Patterns) positive look-behind assertion operator:
    $s =~ s{ addr:0x \K [[:xdigit:]]+ }{$address}xms;
This allows the positive look-behind pattern to be held in a variable and managed separately:
    my $tag = qr{ addr:0x }xms;
    $s =~ s{ $tag \K [[:xdigit:]]+ }{$address}xms;
(Update: The advantage of such separate management is that a more complex pattern may be built up:
     my $tag = qr{ (?: addr | foo | bar) :0x }xms;)

Thanks for the detailed explanation

Hello syedasadali95,

I suppose you want to match any single integer from 0-9 after addr:0x*? If this is the case why not simply like:

$line1 =~ s/addr:0x[0-9]/addr:$address/;
[download]

Also there is no need to escape semicolon character. :)

Update: Fellow Monk AnomalousMonk has a point. This is the more correct solution $line1 =~ s/addr:0x[[:xdigit:]]+/addr:$address/;. Since this a hex value and not just a digit.

Hope this helps, BR.

Thankyou