How to check specific character occurred only once in java regex

Question

I have alphanumeric string where the only #.- are the allowed special characters

my current regex is ^[a-zA-Z0-9][-#._ a-zA-Z0-9 ]+$

which is accepting string as Exam1,Exam#1,Exam1.1 but is also accepting Exam ##1,Exam 1..1

How can I make it match special character only once

Possible duplicate of [Java: How do I count the number of occurrences of a char in a String?](https://stackoverflow.com/questions/275944/java-how-do-i-count-the-number-of-occurrences-of-a-char-in-a-string) — ctwheels, Apr 12 '18 at 14:25
If you do not want repeating special character allowed, what about this case? For example, `Exam #.1 Exam 1.#1 Exam 1.-1` Is this case all right? — Thm Lee, Apr 12 '18 at 15:17

score 0 · Answer 1 · answered Apr 12 '18 at 14:31

First let me explain you regex

^[a-zA-Z0-9][-#._ a-zA-Z0-9 ]+$

Start with character a-z or A-Z or 0-9 followed by atleast one or more of the followed characters.

So move the special character [-#.] into seperate character class and surrounded it with other characters [-#._ a-zA-Z0-9 ]

^[a-zA-Z0-9][_a-zA-Z0-9 ]+[-#.]?[_a-zA-Z0-9 ]+$

score 0 · Answer 2 · answered Apr 12 '18 at 14:40

I assume you talk mean that their can be only one group of special caracters, the following statement should do the trick:

^[a-zA-Z0-9][a-zA-Z0-9 ]*[-#.]*[a-zA-Z0-9 ]*

I'll break it down:

^[a-zA-Z0-9] means it must start with one alphanumeric character

[a-zA-Z0-9 ]* means there might be following alphanumeric characters

[-#.]* means somewhere in the string, there can be a sequence of special characters

[a-zA-Z0-9 ]* means string might end with a sequence of alphanumeric characters or special characters

The first and second part are close but different to prevent a space to be placed at the beginning.

The fact of wrapping the special characters sequence the same structure with * allows it to be anywhere.

2 Answers2