Regex for not containing consecutive characters

Question

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

The string can only contain underscores and alphanumeric characters. It must

Answer 1

See regex in use here

^[a-z](?!\w*__)(?:\w*[^\W_])?$

• ^ Assert position as the start of the line
• [a-z] Match any lowercase ASCII letter. The code below adds the i (case-insensitive) flag, thus this also matches the uppercase variables
• (?!\w*__) Negative lookahead ensuring two underscores do not exist in the string
• (?:\w*[^\W_])? Optionally match the following
  • \w* Match any number of word characters
  • [^\W_] Match any word character except _. Explained: Match anything that is not not a word character, but not _ (since it's in the negated set).
• $ Assert position at the end of the line

let a = ['somestring', '_not_this_one', 'thisone_', 'neither this one', 'but_this_one', 'this__one_not', 'this_one__yes']
var r = /^[a-z](?!\w*__)(?:\w*[^\W_])?$/i

a.forEach(function(s) {
    if(r.test(s)) console.log(s)
});