How to extract a regex pattern in a file using sed

sed -n ‘s/^.*\(<regex_pattern>\).*$/\1/p’ filename

This sed means substitute the whole line with the ‘\1’ (i.e. the matched pattern) and print the line. So it will print the text (not the whole line) which matches <regex_pattern>.

We can also use perl:   perl -i -p0e ‘s/abc\n efg/opq\n rst/’ filename

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Use sed to replace multiline text in a file

e.g.   sed -i ‘N; s/line1\n  line2/line1/’ /path/to/file

“N;” means multiline sed and “\n” means a NEW_LINE_CHAR

More example on http://www.thegeekstuff.com/2009/11/unix-sed-tutorial-multi-line-file-operation-with-6-practical-examples/

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