[SalesForce] Regex in the validation rule

The logic is over-complicated; the second regex returns false when there are only numbers, but you actually wanted it to return true regardless. You can simplify this formula to:

OR( LEN(custom_text_field__c) <> 15, NOT(REGEX(custom_text_field__c,"^[0-9]+$")))

If you want to allow blank values, you need to add that in, too:

AND(NOT(ISBLANK(custom_text_field__c)),
OR( LEN(custom_text_field__c) <> 15, NOT(REGEX(custom_text_field__c,"^[0-9]+$"))))

In your regex, you're looking for one or more digits (any number of them), followed by one or more instances of any single character (the period) and a digit, followed by a required percent sign. 17.5% should pass - I'm not sure why it's not. But so would 67534&8(9h6%. I think you should separate the max value part from the regex part. Try this:

!REGEX(Text_Field__c, "[0-9]{1,3}(\\.[0-9]{1,2})?%?") || 
    IF(RIGHT(Text_Field__c,1) = "%", 
        VALUE(LEFT(Text_Field__c,LEN(Text_Field__c)-1)) > 100, 
        VALUE(Text_Field__c) > 100)

The regex says to look for at least 1, but not more than 3 digits: [0-9]{1,3}. Then a group of characters consisting of a period (which has to be double-escaped with backslashes, once for Salesforce and again for the regex engine) and 1 or 2 instances of a digit: (\\.[0-9]{1,2}). The ? means that whole group should appear once or not at all. And then %? matches a percentage mark once, or not at all. Then the IF block checks to make sure that the number part is <= 100.