Question

118

Regex for not containing consecutive characters

rated 0 times [ 123] [ 5] / answers: 1 / hits: 17163 / 6 Years ago, wed, february 14, 2018, 12:00:00

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]$

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ignacio

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1 Year ago

answered 6 Years ago monicag · Accepted 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!