JavaScript Alphanumeric Regex and allow asterisk at the start of the string but do not allow asterisk at the last 4 digits of the string

Question

I have this regex ^[a-zA-Z0-9*]+$ for only allowing alphanumeric chars and allow Asterisk(*). But I would like allow asterisk only at the start of the string. But asterisk is not allowed at the last 4 digits of the string.

new RegExp('^[a-zA-Z0-9*]+$').test('test') ---Valid
new RegExp('^[a-zA-Z0-9*]+$').test('test1234') --Valid
new RegExp('^[a-zA-Z0-9*]+$').test('test@#_')--Invalid
new RegExp('^[a-zA-Z0-9*]+$').test('****1234') --Valid
new RegExp('^[a-zA-Z0-9*]+$').test('*tes**1234') --Valid
new RegExp('^[a-zA-Z0-9*]+$').test('test****') --Should be Invalid

"How would I allow Asterisk only at the start of the string?" But if the asterisk presents in any of the last 4 positions then it should be invalid

Your question is ambiguous regarding asterisks between the first and the last four positions. The examples don't make that clear either. What about *1*2345, for example? — Heiko Theißen
– Heiko Theißen, Commented Feb 13, 2023 at 16:18
Yes, it is valid. Asterisk is allowed except for last 4 digits. And I have update that case with example — Eternal Sunshine
– Eternal Sunshine, Commented Feb 13, 2023 at 16:21

Peter Thoeny · Accepted Answer · 2023-02-13 19:15:53Z

2

You can use this regex to allow only alphanumeric chars and asterisk, but no asterisk at the last 4 char positions:

const regex = /^(?:[a-z\d*]*[a-z\d]{4}|[a-z\d]{1,3})$/i;
[
  '1',
  '12',
  'test',
  'test1234',
  '****1234',
  '*tes**1234',
  '*1*2345',
  'test@#_',
  'test****',
  'test***5',
  'test**4*',
  '*3**'
].forEach(str => {
  let result = regex.test(str);
  console.log(str, '==>', result);
});

Output:

1 ==> true
12 ==> true
test ==> true
test1234 ==> true
****1234 ==> true
*tes**1234 ==> true
*1*2345 ==> true
test@#_ ==> false
test**** ==> false
test***5 ==> false
test**4* ==> false
*3** ==> false

Explanation of regex:

^ -- anchor at start of string
(?: -- start non-capture group (for logical OR)
- [a-z\d*]*[a-z\d]{4} -- allow alphanumeric chars and asterisk, followed by 4 alphanumeric chars
| -- logical OR
- [a-z\d]{1,3} -- allow 1 to 3 alphanumeric chars
) -- close group
$ -- anchor at end of string

Not that it is easier to read and more efficient to use /.../ instead of new RegExp("..."). You need the regex constructor only if you have variable input.

answered Feb 13, 2023 at 19:15

Peter Thoeny

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1 Comment

Eternal Sunshine Over a year ago

Thanks. In my case this worked for me. That was a clear and concise explanation. Upvoting this since it passed all my test scenarios

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