Why am I getting the error "missing terminating ] for character class"?

Question

I want to cut a string with brackets using sed.

How to avoid an error if I want to drop a string with [? For example:

$ echo '[Om/mystring' | grep -oP '(?<=[Om\\)\w+'
grep: missing terminating ] for character class

Solution 1

As the error missing terminating ] for character class already says, the problem here has to do with [, which you need to escape. Otherwise, it is understood as a character class by grep.

Also, you are saying //, while you want to use / instead of \ according to your input.

All together, this prints a set of words after [Om/:

$ echo '[Om/mystring' | grep -oP '(?<=\[Om\/)\w+'
mystring

Solution 2

You are getting the error because [ is a special character in regular expression syntax, introducing a character class. To be treated as a string literal, it must be escaped i.e. \[

If you just want to remove the [Om/ prefix, then it's simpler sed if you use a delimiter that doesn't appear in the pattern:

$ echo '[Om/mystring' | sed 's;\[Om/;;'
mystring

Comments

[fedorqui]

You can also add the anchor ^ to make it match just the beginning of the line.

[Uyghur Lives Matter]

I don't think the "g" in "\grep" needs to be escaped.

[JoL]

@cpburnz that's a trick to avoid it being interpreted as a shell alias. I imagine he did it to avoid his own alias, while testing this in the shell.