正则表达式匹配有限长度的字符串，如果存在数字，则不能超过 5 位

Question

im looking for regex that can match string with requirement below.

• must be 5 to 15 characters
• Alphanumeric, can accept fully alphabet, if numeric are present, it must not exceed 5 digit and it can be in anywhere in the string.

Example accepted input

• helloworld
• 123helloworld56
• 1h2e3l4l5oworld
• 12345

if the numeric digit exceeded 5 it shall be rejected. Example rejected input:

• 123456
• 123hello4567

So far i have tried while looking online and done some tweaking, but none work as expected.

^(?=.*\d?.*\d?.*\d?.*\d?.*\d?).{0,15}$
^(?=[a-zA-Z1-9]{5,15}$)[a-zA-Z]{1,15}[1-9]{0,5}$
^(?=.*\d){0,5}.{0,15}$

I have stuck on this for some time now, any help are appreciated!

Answer 1

If there can not be more than 5 digits in total, that means you should not be able to match 6 digits.

You can use a negative lookahead to assert what is on the right can not match 6 digits.

^(?!(?:[^\d\r\n]*\d){6})[a-zA-Z0-9]{5,15}$

Explanation

• ^ Start of string
• (?! Negative lookahead, assert what is at the right is not
  • (?:[^\\d\\r\\n]*\\d){6} Match 6 times any char except a newline or a digit, then match a digit
• ) Close lookahead
• [a-zA-Z0-9]{5,15} Match 5-15 times any of the listed in the character class
• $ End of string

Regex demo

Note that using [1-9] in a character class does not match the 0, and \\d will

---

About the patterns in the question

• ^(?=.*\\d?.*\\d?.*\\d?.*\\d?.*\\d?).{0,15}$

Here, the lookahead will always be true as all the parts in it are optional. It could also match an empty string as the quantifier {0,15} starts at 0, which makes it optional.

• ^(?=[a-zA-Z1-9]{5,15}$)[a-zA-Z]{1,15}[1-9]{0,5}$

The pattern asserts a string with 5-15 times any of the listed in the character class. But the matching starts with 1-15 times a char a-zA-Z followed by matching 0-5 times a digit at the end of the string.

• ^(?=.*\\d){0,5}.{0,15}$

The pattern optionally asserts 0-5 digits which is always true as it is optional. Then it matches 0-15 times any char.