Question

I’m trying to allow only certain words through a regexp filter in Java, i.e.:

Pattern p = Pattern.compile("^[a-zA-Z0-9\\s\\.-_]{1," + s.length() + "}$");

But I find that it allows through 140km/h because forward slash isn’t handled. Ideally, this word should not be allowed.

Can anyone suggest a fix to my current version?

I’m new to regexp and don’t particularly follow it fully yet.

The regexp is in a utils class method as follows:

public static boolean checkStringAlphaNumericChars(String s) {
   s = s.trim();
   if ((s == null) || (s.equals(""))) {
        return false;
   }

   Pattern p = Pattern.compile("^[a-zA-Z0-9\\s\\.-_]{1," + s.length() + "}$");
   // Pattern p = Pattern.compile("^[a-zA-Z0-9_\\s]{1," + s.length() + "}");
   Matcher m = p.matcher(s);
   if (m.matches()) {
       return true;
   }
   else {
       return false;
   }
}

I want to allow strings with underscore, space, period, minus. And to ensure that strings with alpha numerics like 123.45 or -500.00 are accepted but where 5,000.00 is not.

Answer 1

You can just use

public static boolean checkStringAlphaNumericChars(String s) { 
    return (s != null) && s.matches("[\\w\\s.-]+"); 
}

• The short-circuited null check ensures s is not null when you try to do .matches() on it.
• Using \w to look for alphanumerics plus the underscore. tchrist will also be the first to point out this is more correct than [A-Za-z0-9_]
• The + at the very end ensures you have at least one character (ie: the string is not empty)
• There’s no need to use ^ and $ since .matches() tries to match the pattern against the whole string .
• There’s also no need to escape the dot (.) in a character class.

New Demo: http://ideone.com/qraob