Ubuntu – Finding the pattern (ab)* using grep

command linegrep

grep ab* filename.txt

The command above finds lines that start with a and end with any number of b's. For example:

a
ab
abb
abb

But how do I fix my command so it finds lines like these?

ab
abab
abababab

I tried grep (ab)* filename.txt, but it won't do it because it's the same as (ab | (ab) | (ab)).

Best Answer

grep (ab)* filename.txt
The one above won't do it because it's the same as (ab | (ab) | (ab)).

No, it won't work because () is special for most shells and you'll get a syntax error:

$ grep (ab)* foo
bash: syntax error near unexpected token `('

Quote your expressions.

Now, grep uses Basic Regular Expressions (BRE) by default. To group part of an expression in BRE, you need to use \(...\):

$ echo xababx | grep -o '\(ab\)*'
abab

Or use Extended Regular Expressions (ERE) or Perl-Compatible Regular Expressions (PCRE), where (...) is enough:

$ echo abab | grep -Eo '(ab)*'
abab
$ echo abab | grep -Po '(ab)*'
abab

See grep manual for BRE vs ERE.

Related Solutions

Ubuntu – Using grep to print line numbers

You can use sed along with grep to achieve this.

grep -n 'Captain' sometextfile.txt | sed 's/:/ /'

This sed command finds the pattern : and replaces it with (a space).

General Syntax - sed 's/find/replace/'

This method will replace only the first occurence of : in each line with . Therefore, even if a line in the text file contains :, it will remain unaffected.

Ubuntu – What’s wrong with the grep command

From man grep (emphasis mine):

grep  searches the named input FILEs (or standard input if no files are
named, or if a single hyphen-minus (-) is given as file name) for lines
containing  a  match to the given PATTERN.  By default, grep prints the
matching lines.

If you ran grep without a filename, or a pipe (the | in ls | grep):

grep foo

The standard input is the terminal - i.e., you. You have to provide the input which grep will search.

$ grep foo
this is me typing stuff
now I'm typing foo, which will get printed again because grep matched foo     
now I'm typing foo, which will get printed again because grep matched foo

Best Answer

Quote your expressions.

Related Solutions

Ubuntu – Using grep to print line numbers

Ubuntu – What’s wrong with the grep command

Related Question