Javascript equivalent of Java Regex

Question

I am trying to write a javascript REGEX to validate a string.

It should validate to the requirement which is as follow.

it should have only Uppercase and lowercase English letters (a to z, A to Z) (ASCII: 65 to 90, 97 to 122) AND/OR Digits 0 to 9 (ASCII: 48 to 57) AND Characters - _ ~ (ASCII: 45, 95, 126). Provided that they are not the first or last character. It can also have Character. (dot, period, full stop) (ASCII: 46) Provided that it is not the first or last character, and provided also that it does not appear two or more times consecutively.

I have a Java version of the REGEX as following.

^(?=[^\W_])[\w~-]++(\.[\w*~-]++)*+(?<=[^\W_])$

And unsuccessfully trying to convert it be used with Javascript. Below is my failed attempt.

^(?=[^\W_])[\w~-]+(.[\w~-]+)(.[\w])$

it fails on the follow two tests

abc..def 1

Test cases for invalid strings:

"a." "1." "a1." "aB78." "aB78..ab" "aB78,1" "aB78 abc" ".Abc12"

Test cases for valid strings:

"a" "1" "a1" "a.1" "abc-def" "a1b2c~3" "012_345"

js regex is mostly the same as any other regex. what have you tried? — llamerr
– llamerr, Commented Mar 21, 2016 at 17:03
What did you try to convert it to JS? Show us your unsuccessful attempt! — Bergi
– Bergi, Commented Mar 21, 2016 at 17:03
^(?=[^\W_])[\w~-]+(\.[\w~-]+)*(.*[a-zA-Z0-9])$ is my unsuccessful attempt. It failes on the abc...def — A Gilani
– A Gilani, Commented Mar 21, 2016 at 17:12

Jan · Accepted Answer · 2016-03-21 18:59:14Z

3

Based on your requirements, you could come up with:

^(?!.*\.{2})(?=.*[a-zA-Z0-9]$)[a-zA-Z0-9][-~.\w]*$

In multiline mode, see a demo on regex101.com.

Explanation:

^ assure it's the beginning of the line
(?!.*.{2,}) negative lookahead - no two dots consecutively
(?=.*[a-zA-Z0-9]$) the last character needs to be one of these
[a-zA-Z0-9] first character needs to be one of these
[-~.\w]* anything of these afterwards (zero or more times)
$ assure the string ends here

edited Mar 21, 2016 at 18:59

answered Mar 21, 2016 at 17:09

Jan

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5 Comments

A Gilani Over a year ago

This works perfectly. Thank you for taking time to explain the regex as well.

A Gilani Over a year ago

The expression fails for a single character and 2 character string. e.g a 1 a1 1a

Jan Over a year ago

@AGilani: Please clarify your requirements then and let's see what we can do.

A Gilani Over a year ago

Please accept apologies for not asking the question in full details. Just added new items to passing and failing test scenarios. Appreciate your help.

Jan Over a year ago

@AGilani: Updated the answer, see regex demo. Using multiple lookaheads now.

ndnenkov · Accepted Answer · 2016-03-21 17:06:52Z

2

There are no lookbehinds in JavaScript regexes. Fortunately, you didn't need it in your case anyway. Replace

(?<=[^\W_])

With just

[^\W_]

answered Mar 21, 2016 at 17:06

ndnenkov

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1 Comment

A Gilani Over a year ago

i came to the same conclusion and rewrote it as follow ^(?=[^\W_])[\w~-]+(\.[\w~-]+)*(.*[a-zA-Z0-9])$

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