Doing a find/replace on the nth line

jdev20 has asked for the wisdom of the Perl Monks concerning the following question:

Hi all I have a shell script that uses perl to do a find/replace on a file, like this

perl -pi -e "s/searchpattern/replacepattern/" filename

My problem is that I only want this to occur at the nth occurence of the file, like this:

junk
junk
replace pattern (this needs to be replaced)
replace pattern
junk

Is there a perl setting that I can use so that only "replace pattern (line 3)" is used, and the "replace pattern" on line 4 is ignored?
Thanks
Joe

Comment on Doing a find/replace on the nth line

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Re: Doing a find/replace on the nth line by BrowserUk (Patriarch) on Oct 05, 2004 at 20:16 UTC
for just the third line `perl -pi -e "$.== 3 and s/searchpattern/replacepattern/" filename` [download] Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l]
Re: Doing a find/replace on the nth line by Limbic~Region (Chancellor) on Oct 05, 2004 at 20:03 UTC
jdev20, You can use $. (see perldoc perlvar) and the modulus operator (% - see perldoc perlop). `perl -pi -e "s/searchpattern/replacepattern/ if $. % 3 == 0" filename` [download] That will apply the search/replace on every 3rd line. Cheers - L~R	[reply] [d/l]
Re: Doing a find/replace on the nth line by ikegami (Patriarch) on Oct 05, 2004 at 21:15 UTC
You've asked to substitue the nth occurance, and you've asked to substitute the occurance on the nth line. You've been provided with a solution the latter; what follows is solution to the former. `perl -pi -e "s/searchpattern/replacepattern/ if (/searchpattern/ && ++$match==3)" filename` [download] hum, let's eliminate the redundancy: `perl -pi -e "substr($_,$-[0],$+[0]-$-[0], 'replacepattern') if (/searchpattern/ && ++$match==3)" filename` [download]	[reply] [d/l] [select]