php - Regular Expression to catch a sequence of letter and digit but containing minimum 3 letters ? how to do? -

I have to make a regular expression in PHP which captures a text that contains the string in which only uppercase letters, numbers, or Underscores are. Example: THIS_STUFF , THIS_2_STUFF , ONE_MORE_STUFF_2_CATCH etc ... This works well with: / [A-Z0- 9_] /

However, I want to capture those sequences which are only:
- Minimum 5 characters ==> / [A-Z0- 9_] { 5,} /
- And with minimum 3 uppercase letters ==> ????

Do you have any ideas?

Thank you!

and with minimum 3 uppercase letters ==> ????

Use at the beginning of this.

  ^ (? = (?: [^ Az \ n] * [az]) {3}) [A-Z0- 9 _] {5,} $  < / Pre> 
 OR 
 
  \ b (? = (?: [^ AZ] * [as]) {3}) [A-Z0-9_] {5,} + \ B