Set limit to string in regular expression with white spaces

Question

I have this regular expression

/^([a-zA-Z01-9]+ ?){0,20}$/

I have a set of rules

• The string line can't start with a white space.
• The string cannot be longer than 20 characters including the white spaces .
• The string cannot have two white spaces consecutively.
• The string can ends with a white space.

The only problem that I have is to set the length of the string including the white space to 20 as maximum.

Answer 1

试试这个： ^(?=[a-zA-Z\\d\\s]{0,20}$)([a-zA-Z01-9]+ ?)*$