How regular expression OR operator is evaluated

If you are interested in what happens when you use | alternation operator, the answer is easy: the regex engine processes the expression and the input string from left to right.

Taking the pattern you have as an example, ^.{8}|.{12}$|.{4} starts inspecting the input string from the left, and checks for ^.{8} - first 8 characters. Finds them and it is a match. Then, goes on and finds the last 12 characters with .{12}$, and again there is a match. Then, any 4-character strings are matched.

Debuggex Demo

Next, you have ^.{8}|.{4}|.{12}$. The expression is again parsed from left to right, first 8 characters are matched first, but next, only 4-character sequences will be matched, .{12} won't ever fire because there will be .{4} matches!

Debuggex Demo

Your Regex ^.{8}|.{12}$|.{4} evaluates to:

Starting with any character except \n { Exactly 8 times }

OR any character except \n { Exactly 12 times }

OR any character except \n { Exactly 4 times } globally

This means that anything after 4 characters in a row will be matched because somewhere in a string of >4 characters there are 4 characters in a row.

1 [false]

12 [false]

123 [false]

1234 [true]

12345 [true]

123456 [true]

1234567 [true]

12345678 [true]

123456789 [true]

1234567890 [true]

12345678901 [true]

123456789012 [true]

You might be looking for:

^.{8}$|^.{12}$|^.{4}$

Which gives you:

1 [false]

12 [false]

123 [false]

1234 [true]

12345 [false]

123456 [false]

1234567 [false]

12345678 [true]

123456789 [false]

1234567890 [false]

12345678901 [false]

123456789012 [true]