A Regex to match any sentence but avoiding character repetition

Question

I am looking for help to create a Regex to validates a string that satisfies these 4 criteria:

1. The string must contain white space.
2. The string must end with a full-stop/period.
3. The string must have a minimum of 15 characters.
4. The string must not contain a repetition of 3 or more characters. This is part I'm particularly struggling with.

For example:

Pass

• One two three four.

Fail

• OOOne two thee four.
• One two thee fffour.
• One two three four
• One two three.
• Onetwothreefour.

If possible could you explain how the regex is constructed to give me a better idea of how to construct them in future?

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Here is what I've tried so far,

^(.*?={15,})(\.\Z)$

but I fear its not even close, please help.

Answer 1

You can use this regex:

^(?=\S*\s)(?!.*?(.)\1{2}).{14,}\.$

RegEx Demo

Explanation:

• (?=\S*\s) - Lookahead to make sure there is at least one whitespace
• (?!.*?(.)\1{2}) - Negative Lookahead to make sure there is no case of 3 consecutive characters
• .{14,} to make sure there are at least 14 characters (15th being the last dot)
• \.$ to make sure dot is always the last character