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)
});

Answer 2

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!

Answer 3

You can also use

^[a-zA-Z]([a-zA-Z0-9]|(_(?!_)))+[a-zA-Z0-9]$

Demo

The only change comparing to your regex is changing [a-zA-Z0-9_] to [a-zA-Z0-9]|(_(?!_)). I removed underscore from the character set and allow it in the second part of the alternative if it's not followed by another one.

(?!_) is negative lookahead meaning that _ cannot be next character