What is DFA?

DFA stands for Deterministic Finite Automaton. Its job is to read in a string of symbols (input) and move (transition) between its states based on the input and ultimately accept or reject the string of symbols

What is Regex and what they do?

Regular expressions are a way of expressing regular languages. In fact, the basic operation of a regex engine is to convert a regex pattern to either an NFA or DFA internally. Then, faced with a string, the N/DFA reads the symbols one by one and transitions between its states and finally spits out if the string of symbols matches the pattern/language or not.