Grep and regular expressions (consecutive letters)

Question

I'm trying to figure out how to list consecutive letters in grep. For example if I wanted to list all words with two consecutive a’s OR i’s OR u’s in one line how would I do this?

From my understanding the command would look something like egrep [a]{2} | egrep [i]{2} | egrep [u]{2}

But what if it's the case that the word has aa and ii or any other combination of the three letters consecutively by twos?

Answer 1

What you've got in your question is actually three separate commands: it'll search for anything that has aa, then pass those results to the next egrep which searches for anything that has ii, then passes those results to the last egrep which searches for uu. So you wind up with only those lines that contain all three of the combinations aa, ii, and uu.

You'd need to use just one egrep command, using a regular expression with alternation:

egrep 'aa|ii|uu'

This will match all lines that contain any of aa, ii, or uu.

Answer 2

You can't. Regex only works with regular expressions. You will notice that regular expressions do not have any sort of memory (thus can't say (.*)*2 or something like that). You're looking for a finite atomota or a turring machine.