[SOLVED] Pattern matching... Is there a clever RegEx?

danielbmartin · 02-20-2015, 09:36 AM

This problem is purely for amusement and self-education.

Read an English-language dictionary and identify words
which fit this pattern:
- the length is 6 characters
- the 3rd and 4th letter are the same but do not appear elsewhere in the word
- the 5th and 6th letter are the same but do not appear elsewhere in the word

I wrote an awk which performs this task.

Code:

awk -F "" '{if (length($0)==6   &&
                        $3==$4  &&
                        $5==$6  &&
              index($0,$3)==3   &&
              index($0,$5)==5) print}' $InFile >$OutFile

There are several words which fit the pattern.
Familiar words such as coffee and toffee;
less-familiar words such as puttee and suttee.

Is it possible to do the same thing with grep using a clever Regular Expression?

Daniel B. Martin

smallpond · 02-20-2015, 10:24 AM

Partial:

Code:

echo coffee |grep -E '^..([a-z])\1([a-z])\2$'
coffee
echo banana |grep -E '^..([a-z])\1([a-z])\2$'

Edit: this doesn't check the part about not appearing elsewhere. Just the two doubled letters.

danielbmartin · 02-20-2015, 10:52 AM

Quote:

Originally Posted by smallpond

Partial:

Code:

echo coffee |grep -E '^..([a-z])\1([a-z])\2$'
coffee
echo banana |grep -E '^..([a-z])\1([a-z])\2$'

Edit: this doesn't check the part about not appearing elsewhere. Just the two doubled letters.

Thank you, smallpond, for this contribution. It works as you said, finding coffee and toffee but also settee and tattoo.

Maybe this can't be done with one grep. Could it be done with two?

Daniel B. Martin

pan64 · 02-20-2015, 11:01 AM

grep -E '^..([a-z])\1([a-z])\2$' | grep -E -v '^.?(.).*\1'

danielbmartin · 02-20-2015, 11:29 AM

Quote:

Originally Posted by pan64

grep -E '^..([a-z])\1([a-z])\2$' | grep -E -v '^.?(.).*\1'

Excellent! Thank you!

Daniel B. Martin

millgates · 02-20-2015, 11:35 AM

Would be easier if grep supported lookaheads like perl does:

Code:

perl -ne '/\b(.)(.)(?!.{0,3}(\1|\2))(.)\4(?!.{0,1}\4)(.)\5\b/ && print'

pan64 · 02-20-2015, 11:37 AM

if you were so kind:
grep -P '\b(.)(.)(?!.{0,3}(\1|\2))(.)\4(?!.{0,1}\4)(.)\5\b'
https://regex101.com/r/nV0qC0/1

smallpond · 02-20-2015, 11:43 AM

The output still differs from the awk expression for these words, if it matters:

Code:

abcccc
aabbcc

millgates · 02-20-2015, 11:44 AM

Quote:

Originally Posted by pan64

if you were so kind:
grep -P '\b(.)(.)(?!.{0,3}(\1|\2))(.)\4(?!.{0,1}\4)(.)\5\b'

I read through the grep man page looking for this two times! How come I missed it?

danielbmartin · 02-20-2015, 11:57 AM

Quote:

Originally Posted by pan64

grep -P '\b(.)(.)(?!.{0,3}(\1|\2))(.)\4(?!.{0,1}\4)(.)\5\b'

Superb! Let's mark this thread SOLVED!

Daniel B. Martin