Grep，找到具有精确匹配模式数的行

Question

I want to find and list lines in text file that contain only two words that are four characters or more.

I can find words of four characters or more with:

grep '[A-Za-z][A-Za-z][A-Za-z][A-Za-z][A-Za-z]*' file.txt

but how can I limit output to show only lines with two such words?

Any hints (not necessarily an answer)?

thanks

UPDATE: Thank you. After following your advice I'm now with:

egrep '([A-Za-z]){4,}' file.txt

That lists all the lines with highlighted words that are 4+ letters long. Now I have only to filter it to show only the lines where such words (4+ letters long) occur twice. Any hints?

Answer 1

To look for two instances of PATTERN , use:

PATTERN.*PATTERN

If you use grep -E you could use curly braces to avoid repetition:

grep -E '(.*PATTERN){2,}'

(You could also apply the same trick to avoid repeating [A-Za-z] in your pattern.)

You can use \\< and \\> to match the beginning and end of words to make sure 8-letter words aren't detected as two 4-letter words.

Answer 2

Just use awk so you don't have to come up with some convoluted regexp to do everything at once. With GNU awk for word boundaries and assuming your "words" only contain alphabetic characters as in your posted script:

awk 'gsub(/\<[[:alpha:]]{4,}\>/,"&") == 2'

The above is untested, of course, since you didn't provide sample input/output for us to test against.

EDIT: Here's the solution given on page 216 in the text you referenced in your comments to exercise 7.5 on page 100 which you based your question on:

egrep '(\<[A-Za-z]{4,}\>).*\<\1\>' file

Let's first clean that up to remove the deprecated egrep and replace the character lists with a portable character class:

grep -E '(\<[[:alpha:]]{4,}\>).*\<\1\>' file

Now what you have is a script that rather than looking for lines that contain only two words that are four characters or more as stated in your question, looks for lines that contain the same 4-or-more character word occurring at least two times which is a very different and much simpler problem to solve.

Answer 3

1st: I recommend using \\w (letter) for letter, it's cleaner.
2nd: To group your pattern into a single token use () to find multiple copies of a regex token use {} . (see Cheat sheet)
3rd: In this case your delimiter is whitespace so I'd use \\s since I assume you might want to catch things like tabs. But that's at your own discretion.

Side note: I recommend avoiding * unless you have a strong delimiter (eg .* will greedy match to the end of your string).

Cheat sheet: https://www.rexegg.com/regex-quickstart.html