Question

What is a regular expression to allow letters only, with no spaces or numbers, with a length of 20 characters?

Some examples of acceptable usernames:

ask1kew
supacool
sec1entertainment
ThatPerson1
Alexking

Some examples of unacceptable usernames:

No_problem1
a_a_sidkd
Thenamethatismorethen20characterslong

Answer 1

This should work if you’re limiting yourself to ASCII:

/\A[a-z0-9]{,20}\z/i

That will also match an empty string though so you might want to add a lower limit (5 in this example):

/\A[a-z0-9]{5,20}\z/i

If you wanted to be adventurous and allow non-ASCII letters and you’re using Ruby 1.9 then you could use this:

/\A\p{Alnum}{5,20}\z/
/\A\p{Alnum}{,20}\z/   # If no lower limit on length is desired.