[SOLVED] Java limit user input with regex

Java limit user input with regex

I need to limit user input of arithmetical operation with Roman numerals. The only possible input is [I to X][+-/*][I to X].

That is what I have:

boolean correctRoman = operation.matches("^(X|IX|IV|V?I{0,3}) ?[+-/*] ?(X|IX|IV|V?I{0,3})$")

What I get for different inputs:

"III"  //false
"III + "  //true-WHY??
"III + X"   //true as it should be

Thank you!

Solution

In this part of the pattern (X|IX|IV|V?I{0,3}) the whole last alternative is optional due to the question mark and the quantifier {0,3} allowing to match "III + "

Note to put the - at the end of the character class or else it will match a range +-/

You can add another alternation to match I II and III and don't make the V optional.

^(X|IX|IV|VI{0,3}|I{1,3}) ?[+/*-] ?(X|IX|IV|VI{0,3}|I{1,3})$

^ Start of string
(X|IX|IV|VI{0,3}|I{1,3}) Match roman numerals 10, 9, 4, 5-8 and 1-3
?[+/*-] ? Match one of the characters between optional spaces
(X|IX|IV|VI{0,3}|I{1,3}) The same number numbers as the first part
$ End of string

Regex demo