how to limit, characters between a range using regular expression - regex

Question

As far as I know {} curly braces are used to limit characters in regular expression like {3,12}, would match character length between 3 to 12.
I am trying to validate username that might contain a period . or _ either one, but not both, doesn't matter placement. For this below regex is working very well.
(^[a-z0-9]+$)|(^[a-z0-9]*[\.\_][a-z0-9]*$)
But I also need to limit the string length between 3 to 12, I had tried to put {3,12} in regex, but that doesn't work.
((^[a-z0-9]+$)|(^[a-z0-9]*[\.\_][a-z0-9]*$)){3,12}
See Example: https://regex101.com/r/kN3aO1/1

Answer 1

As hwnd suggested, a simpler solution would be:
^(?=.{3,12}$)[a-z0-9]+(?:[._][a-z0-9]+)?$
Old solution, which is rather complex and convoluted,is left here for reference, but use the one above instead.
^(?!(?:.{13,}|.{1,2})$)(?:([a-z0-9]+)|([a-z0-9]*[\.\_][a-z0-9]*))$
You can add a lookahead for this.
Demo on regex101

Answer 2

I would do this in three steps.
Check to see if the string has any '/' in it.
Check to see if the string has any '_' in it.
Check to see if string length is between 3 and 12.
In Perl:
if ( ( ( $name =~ /_/ ) && ( $name =~ /\./ ) ) ||
( length($name) < 3 ) ||
( length($name) > 12 ) )
{
# Handle invalid username
}
If you want to make sure that the username contains only one dot or underscore, you may count them. Again, in Perl:
my $dcnt = $name =~ tr /././;
my $ucnt = $name =~ tr /_/_/;
if ( ( $dcnt > 0 && $ucnt > 0 ) ||
( $dcnt > 1 ) ||
( $ucnt > 1 ) ||
( length($name) < 3 ) ||
( length($name) > 12 ) )
{
# Handle invalid username
}
Why not one monster regular expression that does everything at once? Well, for the sake of maintainability. If you or a colleague looks at this code in a year's time, when requirements have changed, this approach will make it easier to update the code.
Notice also that {3,12} says nothing about lengths. It allows the previous pattern to match three to twelve times.