How can I get a list of four letter words using the grep command in the Linux terminal?

Question

I need to get the number of four letter words using the grep command in the Linux shell. My idea was to create a list of four letter words and then use a pipe with | wc -l | wc -l .

I'm pretty new to Linux, but I have tried the following:

cat your_file | grep -c '^[ \t]*[a-zA-Z]\{5\}[ \t]*$'

and

grep -o -w "\w\{5\}" your_file

Answer 1

Use this Perl one-liner:

perl -lne 'print for /\b([A-Za-z]{4})\b/g' in_file

Example:

echo 'ABCD abcd abcd1 abcd_ Abcd,Abcd.' | perl -lne 'print for /\b([A-Za-z]{4})\b/g'

Output:

ABCD
abcd
Abcd
Abcd

The Perl one-liner uses these command line flags:
-e : Tells Perl to look for code in-line, instead of in a file.
-n : Loop over the input one line at a time, assigning it to $_ by default.
-l : Strip the input line separator ( "\\n" on *NIX by default) before executing the code in-line, and append it when printing.

[A-Za-z]{4} : Any 4 letter word = a letter, uppercase or lowercase, exactly 4 occurrences.
([A-Za-z]{4}) : The above, parenthesis used to capture the 4 letter word.
\\b([A-Za-z]{4})\\b : The above, flanked by a word boundary \\b on both sides, which makes it a separate word.
print for /(...)/g : iterate over the captured patterns and print all occurrences.

The regex uses this modifier:
/g : Multiple matches.

SEE ALSO:
perldoc perlrun : how to execute the Perl interpreter: command line switches
perldoc perlre : Perl regular expressions (regexes)
perldoc perlre : Perl regular expressions (regexes): Quantifiers; Character Classes and other Special Escapes; Assertions; Capture groups
perldoc perlrequick : Perl regular expressions quick start