Regex search for word roots with common prefixes

Question

I recently had a curiosity about words in the dictionary that share both "pro-" and "con-" as a prefix. So, for example, procession/concession, produce/conduce, profess/confess, progress/congress, and so on. I'm basically looking for any words that match both ^pro(.+)$ and ^con(.+)$, where the content of the capture group is the same.

My initial caveman command was:

sed -nr 's/^con(.+)$/\1/Ip' /usr/share/dict/words | \
xargs -I SUFFIX -n1 grep -i '^proSUFFIX$' /usr/share/dict/words

It seems to work, outputting a full "con-" word as long as there exists a matching "pro-" word. Problem is, it's slooow. It invokes grep for every potential match, asking it to scan the whole dictionary each time. I could speed it up by making a temporary file that only has pro/con words in it, but it feels like there must be some efficient way to do this without writing a file.

Is there a tool in the GNU world that's well suited to this kind of intersection search?

Answer 1

From my earlier comment to the question itself:

egrep '^(pro|con).* /usr/share/dict/words | sed -nE 's/^(pro|con)(.*)/\2/p' | sort | uniq -d 

will give you a list of all the word-bases that have both a pro and con prefix:

The initial egrep grabs all the words with pro and con prefixes. We then use sed to strip off pro and con from the beginning of each word, sort the list, and then use uniq -d to show ony entries on the list that have duplicates.

Answer 2

This will print out the words without the pro|con prefix:

grep '^\(pro\|con\)' /usr/share/dict/words | cut -c 4- | sort | uniq -c | awk '$1 == 2 {print $2}'

Answer 3

In this particular case - sorted input, so all con... words are listed before pro... words - you could use awk to store the lines matching ^con in an array and when you reach the lines matching ^pro, replace pro with con and if the result is in array print the root word:

awk '/^con/{arr[$0]=$0}; /^pro/{c=gensub(/pro/, "con", 1)
if (c in arr) print substr(c, 4)}' /usr/share/dict/words

.....
.....
vince
vinces
vocation
vocation's
vocations
voke
voked
vokes
voking