I can think of two approaches off the top of my head, other monks will likely have MTOWTDI:

1. Read multiple lines at a time (if your files aren't too big, just read the whole file), write a regex that matches your pattern and also captures the lines before and after your pattern. Writing a regex that matches multiple lines is not too difficult once you've read about the following topics in perlre: the /m and /s modifiers, and the exact meaning of ^, $, ., \s and \n. Also, see I'm having trouble matching over more than one line. What's wrong?

Here's a somewhat inelegant regex that captures the lines before and after a match:

my $input = "line1\nline2\nline3 foo\nline4\nline5";
my ($before,$match,$after) = $input=~/^ (?:(.*)\n)? (.*foo.*) (?:\n (?
+:(.*)\n?)? )? /xm;
print "before=<$before>, match=<$match>, after=<$after>\n";

[download]

2. Keeping a buffer of lines, i.e. an array which always contains the most recent N lines. Such an array could be managed via push and shift. In other words, a sliding window of sorts. This approach would probably be considered less "perlish" than the first, but might be more efficient on large files. Actually, Tie::File may be better than managing the array yourself.

InfiniteSilence just posted an answer that uses the array approach.

In reply to Re: Grab 3 lines before and 2 after each regex hit by Anonymous Monk
in thread Grab 3 lines before and 2 after each regex hit by HarryPutnam

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