mjhjmtu

What is difference between [-a-z] and [a-z] in regular expression?
What is the first minus meaning in [-a-z]?

From default /etc/adduser.conf(For example):

#check user and group names also against this regular expression.`
#NAME_REGEX="^[a-z][-a-z0-9_]*$" 

The hyphen has only a special meaning when it's not the first or last character in a bracketed character class.

[a-z] matches all ASCII lower-case letters from a to z. [-a-z] matches all ASCII lower-case letters from a to z and the hyphen -.

This is documented in the Perl documentation:

Within a list, the "-" character specifies a range of characters, so that a-z represents all characters between "a" and "z", inclusive. If you want either "-" or "]" itself to be a member of a class, put it at the start of the list (possibly after a "^" ), or escape it with a backslash. "-" is also taken literally when it is at the end of the list, just before the closing "]" .

The minus at the beginning is used to include the literal minus character in the group. It might also be at the end of the group. Compare:

$ echo abc- | grep [w-x]

$ echo abc- | grep [w-x-]
abc-
$ echo abc- | grep [-w-x]
abc-

When it's not at the beginning or at the end, it's function is to describe a range, as in w-x.