Bash [[ pattern matching not working

Question

The Bash reference section on conditional constructs says:

When the ‘==’ and ‘!=’ operators are used, the string to the right of the 
operator is considered a pattern and matched according to the rules described 
below in Pattern Matching, as if the extglob shell option were enabled. ...
An additional binary operator, ‘=~’, is available, with the same precedence as 
‘==’ and ‘!=’. When it is used, the string to the right of the operator is 
considered an extended regular expression and matched accordingly (as in regex 
3)). 

But then I try this:

$ [[ -good == -* ]] ; echo $?
0
$ [[ -g == -* ]] ; echo $?
bash: syntax error in conditional expression
bash: syntax error near `-*'
$ [[ -g == -? ]] ; echo $?
bash: syntax error in conditional expression
bash: syntax error near `-?'

Then I try the regex matching operator =~:

$ [[ -good =~ -.* ]] ; echo $?
0
$ [[ -g =~ -.* ]] ; echo $?
bash: syntax error in conditional expression
bash: syntax error near `-.*'
$ [[ -g =~ -.? ]] ; echo $?
bash: syntax error in conditional expression
bash: syntax error near `-.?'

Why the syntax errors?

Answer 1

Well you have to be careful that the text to the left of ==, =~ or != is not one of the operators recognized in Bash conditional expressions.

In your case, -g tests for a setgid bit set. If you had provided something that is not recognized as an operator in this context, then it would work:

$ [[ -i == -* ]] ; echo $?
0
$ [[ -i == -? ]] ; echo $?
0
$ [[ -i =~ -.* ]] ; echo $?
0
$ [[ -i =~ -.? ]] ; echo $?
0

One way to reliably provide any string would be to quote it, so it is recognized not as an operator but as just a string:

$ [[ "-good" == -* ]] ; echo $?
0
$ [[ "-g" == -* ]] ; echo $?
0
$ [[ "-g" == -? ]] ; echo $?
0
$ [[ "-good" =~ -.* ]] ; echo $?
0
$ [[ "-g" =~ -.* ]] ; echo $?
0
$ [[ "-g" =~ -.? ]] ; echo $?
0

However take care not to quote the right hand side of the operator since it will prevent that from being recognized as a pattern:

$ [[ "-good" == "-*" ]] ; echo $?
1
$ [[ "-g" == "-*" ]] ; echo $?
1
$ [[ "-g" == "-?" ]] ; echo $?
1
$ [[ "-good" =~ "-.*" ]] ; echo $?
1
$ [[ "-g" =~ "-.*" ]] ; echo $?
1
$ [[ "-g" =~ "-.?" ]] ; echo $?
1

Answer 2

We should put the string inside quotes to evaluate them as strings. If we try to evaluate a string having embedded hyphen(-) then shell will consider it as an option to test but not as string. For more information on the operators you can check this here