I want to match one or more occurance for ex. of blah using expr.
This works.
$ expr "blahblahblah" : 'blahblah'
8
What is wrong with this regular expression?
$ expr "blahblahblah" : '\(blah\)\+'
blah
I want the number of characters matched.
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2Why is "using expr" a part of the question, rather than "regex-matching a string and knowing how many characters matched"?Charles Duffy– Charles Duffy2012-07-23 16:42:35 +00:00Commented Jul 23, 2012 at 16:42
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expr is an external command, not a part of bash -- if you really want an expr command to do this, it isn't a bash question at all.Charles Duffy– Charles Duffy2012-07-23 16:49:54 +00:00Commented Jul 23, 2012 at 16:49
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3 Answers
Since your question is tagged bash, there are better facilities than expr in modern versions of the shell, which do exactly what you want:
$ re='(blah)+'
$ [[ foo_blahblah_bar =~ $re ]] && echo "${#BASH_REMATCH[0]}"
8
1 Comment
minice
Yes, I am using it in a bash script, so bash only syntax definetely makes sense.
First of all, you need \(\) instead of () and \+ instead of +.
But that is not all.
You can't use groups () and get the length of matched string simultaneously.
Pattern matches return the string matched between ( and ) or null; if ( and ) are not used, they return the number of characters matched or 0.
You must use wc to get the length of the string:
$ expr "blahblahblahblah" : '\(\(blah\)\+\)' | wc -c
17
Or using parameters expansion:
$ m=$(expr "blahblahblahblah" : '\(\(blah\)\+\)')
$ echo ${#m}
16
(wc -c counts the end of line also, hence the difference).
But if you can write the regular expression without groups, you get the length:
$ expr "blahhhhhbl" : "blah\+"
8
5 Comments
Charles Duffy
No reason to use wc -- you can get the length with parameter expansion, which avoids the overhead of forking off a subprocess, setting up a pipeline, capturing its output, etc.
Charles Duffy
...well, I said that before running
type expr; turns out it's an external command, so there's fork+wait+read overhead in running it no matter what.
Igor Chubin
@CharlesDuffy: anyway ${#a} looks better than
wc -c. And your answer ([[ =~ ]]) is even betterminice
Guys thanks, I am impressed! I resort to bash only in seldom need situations, but I am going to be coming back here more often in future.
Igor Chubin
bash is a really mighty thing. I think, you will be excited
Using sed:
echo "blahblahblah" | sed -n 's!\(\(blah\)*\).*!\1!p' | wc -c
