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 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]$

Perhaps, you might write it as `/^[A-Z](?!.*__)(?:\w*[A-Z0-9])?$/i` — Wiktor Stribiżew, Feb 14 '18 at 14:18
If this is targetting passwords, please refer to [Reference - Password Validation](https://stackoverflow.com/questions/48345922/reference-password-validation/) — ctwheels, Feb 14 '18 at 16:44

Jan · Accepted Answer · 2020-05-31T21:35:20.823

10

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!

edited May 31 '20 at 21:35

answered Feb 14 '18 at 13:59

Jan

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Why the `\n` ?? – Exception_al Feb 14 '18 at 14:00
Looks awesome. One error tho: 'this_one__yes' – Mariusz Feb 14 '18 at 14:04

score 3 · Answer 2 · answered Feb 14 '18 at 14:00

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

answered Feb 14 '18 at 14:00

mrzasa

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Does not work with `a` or `b` or `ab`, see https://regex101.com/r/QgbEYV/2 (meaning the string must be at least 3 characters long, dunno if this is a requirement or not). – Jan Feb 14 '18 at 14:08

ctwheels · Answer 3 · 2018-02-14T18:33:17.690

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)
});

Regex for not containing consecutive characters

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