Syntax error =~ operator in msysgit bash

Question

I'm trying to add a function to my bash_profile for msysgit:

function git-unpushed {
    brinfo=$(git branch -v | grep git-branch-name)
    if [[ $brinfo =~ ("[ahead "([[:digit:]]*)]) ]]
    then
        echo "(${BASH_REMATCH[2]})"
    fi
}

But I get the following error:

bash: conditional binary operator expected`

bash: syntax error near =~'

From what I can find, the "equals tilde" operator (=~) evaluates as regex in bash.

Why is =~ is throwing an error?

UPDATE: Here's a screenshot of inputting it manually (this is running sh.exe):

Answer 1

I had the same error on Bash 3.1.0 from Git installation on Windows. Ultimately I changed it to:

if echo $var | grep -E 'regexp' > /dev/null
then
  ...
fi