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I have the following regex so far:

printf "this is (test.com)\n" | grep -Po '(?<=\().*(?=\))'

The above regex will print the desired output, which is test.com

As far as I am adding new parenthesis to my code

printf "this is (test.com) and (alex)\n" | grep -Po '(?<=\().*(?=\))'

, it displays something like:

test.com) and (alex

I would like my regex to print anything between brackets (no matter how many there are). More, I would like to add a condition to print just words between brackets with a specific length.

Eg: (aaa) test1 (bbb) test test (ccc) test (example) (ddd)

I just want printed out the words between brackets that have length 3. So the output should be:

aaa bbb ccc ddd

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    Use [^)] instead . or if you already use -P (pcre regex) to prohibit regex be greedy you should add ? after *: grep -Po '(?<=\().*?(?=\))'
    – Costas
    Feb 11, 2015 at 20:51
  • That worked. What about the lenght condition ? Where should I place it, and how ... in order to print words with only 3 characters ?
    – Grajdeanu Alex
    Feb 11, 2015 at 20:55
  • You have to study quantificator thoroughly. Three characters can be choised like ... (any 3 symbols) or, for example \w{3} (any 3 alphanumeric symbols)
    – Costas
    Feb 11, 2015 at 21:05

1 Answer 1

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Please be noted that expression try to choice the pattern of maximum length (gready regex) match. As you see in your example (regex: symbols between parenthesis) have choiced

  • ...is ( test.com) and (alex )

instead of

  • ...is (test.com) and (alex).

There are two ways to override such behavior:

  1. Substitute any symbol by revers match of limit or devide symbol (for example: (.*) by ([^)]*)
  2. Modern regular expressions (PCRE for example) allow a quantifier to be specified as lazy. They put a question mark after the quantifier to make it lazy .*?. By using a lazy quantifier, the expression tries the minimal match first.

More over the first variant allows to some simplify regex: grep -Po '\(\K[^)]*'

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  • Thank you for your response. I would vote this up but I don't have the necessary rep. Supposing that between paranthesis are just letters, it would be easier to add a length condition ?
    – Grajdeanu Alex
    Feb 11, 2015 at 21:48
  • Try to indicate the exact term for expression: Output words in parenthesis which consist in 3 letter only. Is it correct? '\(\K\w\w\w(?=\))' or '\(\K\w{3}(?=\))'
    – Costas
    Feb 11, 2015 at 21:55
  • It worked. Thank you for your time. I will look into these so that I will have a better understanding into regex conditions
    – Grajdeanu Alex
    Feb 11, 2015 at 22:02

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