In sed, how to represent "alphanumeric or _ or -"

Question

Tried

[a-zA-Z0-9-_]
[a-zA-Z0-0\-\_]
[[[:alnum:]]-_]
...

What is the correct way to represent that in regular expression?

It seems like [a-zA-Z0-9-] works for alphanumeric or dash. But when I add another underscore, it breaks.

Answer 1

That will be this character class:

[[:alnum:]_-]

Which means allow one of these:

1. Alpha numeric
1. Underscore
1. Hyphen

It is important to keep hyphen at 1st or last position in character class to avoid escaping.

Answer 2

All of these variations will work:

[a-zA-Z0-9_\-]
[a-zA-Z0-9_-]
[-_a-zA-Z0-9]
[-a-z_A-Z0-9]
[-a-zA-Z_0-9]
[-a-zA-Z0-9_]
...

The hyphen needs to be on one end. It can be escaped or not if at the end, and must be un-escaped if at the beginning. The underscore can be almost anywhere, as long as it's not in the middle of one of your ranges, and doesn't need to be escaped.