Regex for not containing consecutive characters

I can’t figure out javascript regex that would satisfy all those requirements:

The string can only contain underscores and alphanumeric characters. It must begin with a letter, not include spaces, not end with an underscore, and not contain two consecutive underscores.

This is as far as I came, but ‘not containing consecutive underscores’ part is the hardest to add.

^[a-zA-Z][a-zA-Z0-9_]+[a-zA-Z0-9]$

Answer

You could use multiple lookaheads (neg. ones in this case):

^(?!.*__)(?!.*_$)[A-Za-z]w*$

See a demo on regex101.com.

---

Broken down this says:

^           # start of the line
(?!.*__)    # neg. lookahead, no two consecutive underscores (edit 5/31/20: removed extra Kleene star)
(?!.*_$)    # not an underscore right at the end
[A-Za-z]w* # letter, followed by 0+ alphanumeric characters
$           # the end

---

As JavaScript snippet:

let strings = ['somestring', '_not_this_one', 'thisone_', 'neither this one', 'but_this_one', 'this__one_not', 'this_one__yes']

var re = /^(?!.*__)(?!.*_$)[A-Za-z]w*$/;
strings.forEach(function(string) {
    console.log(re.test(string));
});

Please do not restrain passwords!