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Some English words contain an "abba" structure, such as "tomorrow" or "better". How many lines of a given file contain such a word? How do i search such words using grep?

  • 3
    These seem like homework questions which you aren't really trying to research at all. Try consulting the man page of grep. – cutrightjm Nov 21 '15 at 1:38
  • Can you supply more language and linguistic information about the parameters of the search? I'm not exactly sure what you mean by "abba" structure and how it applies/relates/defines the attributes of "tomorrow" or "better". – RobertL Nov 21 '15 at 1:40
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Try this with GNU grep:

grep '\(.\)\(.\)\2\1' file | wc -l
  • why do you specify GNU grep here? – mikeserv Nov 21 '15 at 8:04
  • @mikeserv: I tested it with GNU grep. – Cyrus Nov 21 '15 at 9:40
  • It is telling me there is no such file in directory. skhalid@cs:~/assignment3$ grep '(.)(.)\2\1' file | wc -l macbeth.html 5243 macbeth.html grep: file: No such file or directory – Usman Nov 21 '15 at 13:02
  • Replace "file" by your filename. – Cyrus Nov 21 '15 at 13:12
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If you are looking for patterns that match palindromes - or words that are spelled the same way both forwards and backwards - you might open a GNU grep's info grep page and find...

Guglielmo Bondioni proposed a single RE that finds all palindromes up to 19 characters long using 9 subexpressions and 9 back-references:

grep -xE '(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?\9\8\7\6\5\4\3\2\1'

Note that this is done using GNU ERE extensions; it may not be portable to other implementations of grep.

...now that will match abba, but not tomorrow or better because neither of those words is a palindrome.

That note on portability, as I think, is specifically referring to the use of back-references in an Extended Regular Expression pattern - you'd need a lot more backslashes and a \{0,1\} to replace every question mark for a fully portable expression, I think.

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