Why does this regex work in grep but not sed?

Question

I have two regular expressions:

1. $ grep -E '\-\- .*$' *.sql
2. $ sed -E '\-\- .*$' *.sql

(I am trying to grep lines in sql files that have comments and remove lines in sql files that have comments)

The grep command works using this regex; however, the sed returns the following error:
sed: -e expression #1, char 7: unterminated address regex

What am I doing incorrectly with sed?

(The space after the two hyphens is required for sql comments if you are unfamiliar with MySql comments of this type)

Answer 1

You're trying to use:

sed -E '\-\- .*$' *.sql

Here sed command is not correct because you're not really telling sed to do something.

It should be:

sed -n '/-- /p' *.sql

and equivalent grep would be:

grep -- '-- ' *.sql

or even better with a fixed string search:

grep -F -- '-- ' *.sql

Using -- to separate pattern and arguments in grep command.

There is no need to escape - in a regex if it is outside bracket expression (or character class) i.e. [...].

---

Based on comments below it seems OP's intent is to remove commented section in all *.sql files that start with 2 hyphens.

You may use this sed for that:

sed -i 's/-- .*//g' *.sql

Answer 2

The problem here is not the regex, the problem is that sed requires a command. The equivalent of your grep would be:

 sed -n  '/\-\- .*$/p' 

You suppress output for non-matching lines -n ... you search (wrap your regex in slashes) and you print p (after the last slash).

P.S.: As Anub pointed out, escaping the hyphens - inside the regex is unnecessary.

Answer 3

You are trying to use sed's \cregexpc syntax where with \-<...> you are telling sed the delimiter character you want use is a dash -, but you didn't terminate it where it should be: \-<...>- also add d command to delete those lines.

sed '\-\-\-.*$-d' infile

see man sed about that:

\cregexpc Match lines matching the regular expression regexp. The c may be any character.

if default / was used this was not required so:

sed '/--.*$/d' infile

or simply:

sed '/^--/d' infile

and more accurately:

sed '/^[[:blank:]]*--/d' infile