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For basic command-line pattern matching, I first try grep. Even though there are several grep utilities (grep, egrep, fgrep), I tend to use the original grep command. While by default it implements clunky POSIX-style pattern matching, its -P argument asks it to honor perl regular expression syntax. In most cases this works well.

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• -v  (inverse) – only print lines with no match
• -n  (line number) – prefix output with the line number of the match
• -i  (case insensitive) – ignore case when matching alphanumeric characters
• -c  (count) – just return a count of the matches-n  (line number) – prefix output with the line number of the match
• -L  – instead of reporting each match, report only the name of files in which a match is found
  • handy for checking a bunch of log files for errors or success
• -A  (After) and -B (Before) – output the specified number of lines before and after a match

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If grep isn't behaving the way I expect, I turn to perl. Here's how to invoke regex pattern matching from a command line using perl:

perl -n -e 'print if $_=~/<some pattern>/;'

# for example:
echo -e "12\n23\n4\n5" | perl -n -e 'print if $_ =~/\d\d/'

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