What is Regular expression engines

The primry work of searching any regular expression pattern is done by Regular expression engines. Regular expression engines have differences based on their type and way of there working. There are mainly two Regular expression engines: Deterministic Finite Automaton (DFA) and Nondeterministic Finite Automaton (NFA). DFAs are faster, but lack many of the features of an NFA, such as capturing, look around, and non greedy quantifiers. The NFA further categories in two sub types: traditional NFA and POSIX.

Deterministic Finite Automaton (DFA) regex engines

DFAs compare each character of the input string to the regular expression, keeping track of all matches in progress. Since each character is examined at most once, the DFA engine is the fastest.

Traditional Nondeterministic Finite Automaton (NFA) engines

Traditional NFA engines compare each element of the regex to the input string, keeping track of positions where it chose between two options in the regex. If an option fails, the engine backtracks to the most recently saved position.

POSIX NFA engines

POSIX NFA Engines work similarly to Traditional NFAs with one exception: a POSIX engine always picks the longest of the leftmost matches. The POSIX standard requires that if you have multiple possible matches that start at the same position, the one matching the most text must be the one returned.