There are different flavors of regular expression implementation. In the end, what engines are doing, they are converting regex patterns into either DFA or NFA (or both). No one wants to end like Cloudflare! There are certain interesting topics of ReDoS (regular expression DoS attacks) or evil regex – patterns which require a lot of unnecessary backtracking. Such extra care should always be invested in pattern optimization and understanding how exactly the engine will handle the tested string.
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There is the possibility to use a tremendous amount of time and resources if a pattern is using too much backtracking. Worth noting is that NFA looks more powerful, since it can incorporate more advanced search patterns, but as always, with great power comes great responsibility. That means that both DFA and NFA can sometimes match different strings using the same pattern. There is even a slight difference in what would be matched, as DFA will always find the longest possible match furthest to the left (longest-leftmost match), whereas NFA would generally tend to find the first match satisfying the conditions. Both differ in the efficiency, implementation complexity and in what they support (for example non-greedy quantifiers). For example, powerset construction can be used for that. One of the properties of NFA is that it can always be converted into the DFA by just expanding the number of states. NFA can even perform a transition on an empty string. When there is more than one transition to a given symbol it branches and the branch lives as long as there is a valid transition. NFA being non-deterministic counterpart can have zero, one or more transitions corresponding to a particular symbol. I sometimes picture DFA as a DFS/BFS algorithm in terms of how states graph is traversed. As it would never test the same character twice (no backtracking is required), it would always run in the linear time, as it takes the same amount of time to confirm whether there is a possible match or not at all. There are two types of finite-state automata used for that purpose:ĭFA will always map a single state to a single symbol in a pattern, which means that its transitions are deterministic. If the end state is reached it means that the given string matches the defined pattern. Implementations of regular expressions are done using an abstract computation model: finite-state machines (finite state automata) which takes a string of symbols and is running through an internal state sequence determined by the given string.
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Regex is a sequence of characters that define a search pattern.
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What is behind the term r egular expression? According to the source of all knowledge, to which everyone refers to (but is afraid to admit it): So let’s jump right into it, shall we? The theory behind it
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Reading about finite automata or Thompson’s construction algorithm could give you a head start in terms of computational linguistics, in case you are interested in knowing how it is possible to create a mathematical notation of a language. The basic ‘*’ is often used as a wildcard, having its base in the Kleene closure, formed by the creator of the concept of r egular expression. Have you ever heard of regular expressions or regex / regexp for short? There is a very high chance that you’ve even used it without knowing.