How to check number of different characters using regex?

Question

I'm trying to create regex to find all inputs containing max three different characters. It doesn't matter how long the input is.

Example of cases:

• "32 32 32 32 34" --> match
• "MM" --> match
• " " --> match
• "1234" --> no match

I've done regex to find inputs of four or more different chars, but now I need it in opposite way...

(.).*(?\1)(.).*(?\1)(?\2)(.).*(?\1)(?\2)(?\3)(.)

Main question is: How to check number of different characters?

Answer 1

The following will match a string with a maximum of three different non-space characters

^\s*(\S)?(?:\s|\1)*(\S)?(?:\s|\1|\2)*(\S)?(?:\s|\1|\2|\3)*$

(\S) matches one non-space character and captures it so it can then be referenced later in the regex using a back-reference e.g. \1. The ? in the (\S)? are used so the string can contain zero, one, two or three types of non-space characters.

The ?: make a group non-capturing.

The first part of the regex captures up to three different non-space characters \1, \2, \3, and then (?:\s|\1|\2|\3)* ensures only those characters or space \s can then appear before the end of the string $.

One way, in Javascript, to count the number of different non-space characters in a string "using regex":

var str = 'ABC ABC';
var chars = '';

str.replace( /\S/g,  function ( m ) {
    if ( chars.indexOf(m) == -1 ) chars += m;
});

chars.length;    // 3