Validate field with Regular Expression

I want to validate 2 fields within my form with Regular Expression
I searched the forum and found some posts that are not clear enough about how to do it, so please give me a "clear" steps on how to validate a specific text input field with my Regular Expression using hooks/filters or whatever it takes, clearly, which part of the code put where?!
Thanks,

Thanks Carl, it worked for me :)
Now, another tiny question...
I am using conditional logic for some fields within my forms its easy and works great, only one case I can't achieve with gravity where:
- Field A is a radio button with 2 choices
- Field B is a drop down menu with 4 values
- Field C is a text input shows ONLY IF (Field A = Choice1) AND (Field B = value1 OR value2 OR value3)
- Field D is a text input shows ONLY IF (Field A = Choice1) AND (Field B = value4)
I can get the Field D to work probably but not Field C hence the lake of 'OR' ability in the conditional logic with gravity, am I missing anything here? is there anyway to do it?

Bad news :(
Thanks anyway Carl, I'll work in a workaround, just put it in your To-Do Features list, gonna be awesome to see such a function in Gravity... Kindly mark this thread as answered and close it, no more question here for now :)