Question

I have a file with following content:

[A hi] [B hello]
[A how] [A why] [C some where]

I basically want to extract the “text” with marker ‘A’ I mean

hi
how
why

in a new file on separate lines.
I tried using sed but I could not get the regular expression. Can someone suggest me what can I use ?

Answer 1

Try doing this using grep :

grep -oP '\[A\s+\K[^\]]+' file.txt > new_file.txt

or

grep -oP '\[A\s+\K[^\]]+' file.txt | tee new_file.txt

RESULT

hi
how
why

EXPLANATIONS

• -o for grep stands for “get only the matching part”
• -P for grep stands for “Perl extented regex”
• for the \K regex trick, see Support of \K in regex (it’s an advanced look-around regex trick)

The same regex in perl with comments :

use strict; use warnings;
use feature qw/say/;

while (<>) {
    say for 
        /           # starting regex
            \[A     # a literal "[" and "A"
            \s+     # at least one whitespace (\n, \r, \t, \f, and " ")
            \K      # restart the match
            [^\]]+  # at least one character that is not a literal "]"
        /gsx;       # end of the regex and the modifiers
}

LINKS

To learn regex, see

• http://www.regular-expressions.info/
• Learning Regular Expressions