UNIX for Dummies Questions & Answers

How do you do grouping in grep? Here's how I tried it at first:

Code:

egrep 'qualit(y|ies)' /usr/share/dict/words
-bash: syntax error near unexpected token `('

I'm using GNUgrep, and I found this on their site. grep regular expression syntax

So I tried this:

Code:

egrep 'qualit\(y\|ies\)' /usr/share/dict/words

And got no results, although clearly these two words are in the dictionary, (yeah, I even checked). Now, I could've sworn I used parentheses in grep for grouping, and the pipe as an "or" operator. So I thought maybe it was GNUgrep that was the problem, so to speak, so I tried the old /bin/grep, but got similar results. Clearly I'm going something wrong. What obvious thing am I missing?

It looks like you're not using the quoting character. It should be ', not ’.

Note that the error is not from egrep; it's from bash. In the second example, since those backslashes do not occur within a quoted string, they are being interpreted by the shell, to whom they quote the following character. The special shell character generating the syntax error in the first example, (, in the second is quoted with a backslash, hence no syntax error.

Also, quoting aside, the second egrep's regular expression is incorrectly specified. Those backslash escaped characters are special by default in egrep's extended regular expression flavor; you only need to quote them if you want to match them literally. In grep's basic regular expression flavor, the situation is reversed: those characters are ordinary by default and need to be backslashed to become special.

Regards,
Alister

That's weird. I'm not even sure how I got that character. Guess I had my finger in the wrong place, once again. ; )
Great job spotting that!

Well that's what I thought; escaping makes them literal. But then I read that bit on the GNU grep page. Maybe that applies to grep rather than egrep?