Question

I have some form numbers I need to validate. I’ve tried multiple attempts but am not getting it right yet. While much is allowed in a form number there are some limits I need to impose:

All of these rules should be enforced:

• A-Z allowed but not required (see bullet 4)
• 0-9 allowed but not required (see bullet 4)
• period (decimal point) and dash, if present, only allowed once per form number . –
• Minimum length is one character and it cannot be a space, dash or period
• multiple spaces are allowed but two spaces may not be next to each other; also no leading or trailing spaces are allowed

This is what I had before but not all the above rules were enforced.

[A-Z0-9]([A-Za-z0-9 -.])*[A-Z0-9]

So these would be examples of valid form numbers under the new requirements:

123 
123 456 
A1 IL 23 MN 
CL-100 2.0 

These would be examples of invalid form numbers under the new requirements:

123   456
25! 25
25-IL 30-1
aa bb CC

Answer 1

This should work

^([A-Z0-9]|(?! )(?!.* $)(?!.*  )(?!.*-.*-)(?!.*\..*\.)(?![.-]$)[A-Z0-9 .-]+)$

There are two parts. The first one [A-Z0-9] checks for a single character. If it isn’t a single character then there are some exclusion rules (?! )(?!.* $)(?!.* )(?!.*-.*-)(?!.*\..*\.)(?![.-]$) (in order): no beginning with space, no ending with space, no consecutive double spaces, no two -, no two ., no single character . or - followed by end-of-string. Then there is the “base” pattern (one or more of) [A-Z0-9 .-]+

Note that you’ll have to escape the \ with another \, so \\.