Why does grep need a dot in a regex using ‘$’

grepregex

I though I knew grep, but maybe not.

I want to find all lines in a file ending with ':' If I run

grep :$ ~/greptester.txt

but to my surprise, it gives no results. Sometimes I confuse '^' and '$', having to guess which is begin and which is end of line, but I checked and $ is indeed the end of a line.

After much screwing around, I accidently discovered that running

grep :.$ ~/greptester.txt

does give the expected results. Why?

Here is the text file:

test line one
1 line with a colon:
ignore this line
3456 some stuff:
cat: meow; dog: bark; horse: four (4) legs.
goat, 7 elephants

This happens both on Ubuntu and on a Windows machine with Cygwin.

Best Answer

Your file has \r\n (CR+LF) line endings (likely, created in windows?), whereas most UNIX files only end in \n (LF).

So, before grep sees the \n after the : character, there is the \r that it has to match with the . wildcard.

Related Solutions

Regex : does not begin by “pattern”

Using grep in this case with -P option, which Interprets the PATTERN as a Perl regular expression

grep -P '^(?:(?!git).)*$' LIST

Regular expression explanation:

^             the beginning of the string
 (?:          group, but do not capture (0 or more times)
   (?!        look ahead to see if there is not:
     git      'git'
   )          end of look-ahead
   .          any character except \n
 )*           end of grouping
$             before an optional \n, and the end of the string

Using the find command

find . \! -iname "git*"

Linux – grep with regex containing pipe character

It appears that grep accepts \| as a separator between alternative search expressions (like | in egrep, where \| matches a literal |).

Apart from that, your expression has other problems:-

+ is supported in egrep (or grep -E) only.
\s is not supported within a [] character group.
I don't see the need for | in the character group.

So the following works for grep:-

grep "{{flag|[a-zA-Z ][a-zA-Z ]*}}" <temp

Or (thanks to Glenn Jackman's input):-

grep "{{flag|[a-zA-Z ]\+}}" <temp

In egrep the {} characters have special significance, so they need to be escaped:-

egrep "\{\{flag\|[a-zA-Z ]+\}\}" <temp

Note that I have removed the unnecessary use of cat.

Best Answer

Related Solutions

Regex : does not begin by “pattern”

Linux – grep with regex containing pipe character

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