How to write regex to enforce maximum of 255 characters?

What regular expression would match any characters (including spaces), but have a maximum of 255 characters? 开发者_开发百科 Is this it?

^[a-zA-Z0-9._]{1,255}$

Well, anything would be:

^.{1,255}$

. doesn't allow new lines. If that's a problem, you can use the dot-all flag (usually /s).

If you want to add spaces to your regex, try this (note the space):

• ^[a-zA-Z0-9._ \t]{1,255}$ - Allow spaces and tabs.
• ^[a-zA-Z0-9._\s]{1,255}$ - Allow all whitespaces.
• ^[\s\w.]{1,255}$ - Same as the above (unless your flavor supports Unicode).

Well that would not allow anything, if you want anything, you're better off using ^.{1,255}$.

Or, if you want to allow nothing as well: ^.{0,255}$