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May 12th, 38 0. Arguably none is more ubiquitous than regular expressions. A regular expression, or regex, is a string that enables a developer to express a pattern being searched for, making it a very common way to search text and to extract from the results key finds. Every major development platform has one or more regex libraries, either built into the platform or available as a separate library, and. NET is no exception. RegularExpressions namespace has been around since the early s, introduced as part of.
NET Framework 1. NET applications and services. At the time it was introduced, it was a state-of-the-art design and implementation. This was rectified in. NET 5, where we re-invested in making Regex very competitive, with many improvements and optimizations to its implementation elaborated on in Regex Performance Improvements in. NET 5. Now with.
NET 7 is an awesome choice for your text searching needs in. There are multiple ways a regex engine the thing that does the actual searching can be implemented.
Since the beginning of. In doing so, it might end up needing to examine the same text multiple times. Such backtracking engines also can be incredibly efficient, in particular when the thing being searched for matches and does so with as few wrong tries along the way as possible.
If you try to match this against the input "12a" ASCII numbers are both digits and word characters , it will:. Now we find it performs roughly as follows:. Notice that by adding one more alternation, we actually doubled the number of steps in our matching operation.
One more, double it again. And so on. And there in lies the rub. We can actually see this in practice. Try running the following code and after starting it, go get a cup of coffee , which is the expression we just talked about, except using a repeater to express multiple alternations rather than copy-and-pasting that subexpression multiple times:.
By the time we get to 30 alternations, what once was fast is now taking more than two and a half minutes. This is the whole reason. Thus, Regex supports timeouts, and guarantees that it will only do at most O n work where n is the length of the input between timeout checks, thus enabling a developer to prevent such runaway execution.
Imagine the regular expression being turned into a graph, where every construct in the pattern is represented as one or more nodes in a graph, and you can transition from one node to another based on the next character in the input.
For example, consider the simple expression abc cd. As a directed graph, this expression could look like this:. If the next character is a 'c' , we transition to node 3. If the next character after that is a 'd' , we transition to the final state of node 4 and declare a match.
To address that, we can prefix the expression with a. If we do that, we get almost the exact same graph, but this time with an extra transition from the start state back to the start state. Note that the transition is tagged as. As noted, this can result in exponential worst case processing time for some expressions.
But there are other ways to process an NFA. For each character in the input we read, we enumerate all the states in our set, and for each, find all the new nodes we could transition to, creating our new set. That makes a DFA really valuable for a regex engine, because it means the engine simply needs to make a single walk through the input at least to determine whether there is a match : read the next character, transition to the next node, read the next character, transition to the next node, and on and on until either a final state is found match or it dead-ends, unable to transition out of the current node for the next input character no match.
This leads to O n worst-case processing time. The graph, however, is considerably more complex:. That means a regex engine using this approach can employ such a graph to determine whether there is a match, but it then needs to do additional work to determine, for example, where the match starts, or the values of any subcaptures that might be in the pattern.
NET 7, developers using Regex now also have a choice to pick such an automata-based engine, using the new RegexOptions. Going back to my previous catastrophic backtracking example, we can change the constructor call from:. On my machine, I see numbers like this:. The processing is now effectively linear in the length of the short input. If I subtly change the original program from doing:. The new RegexOptions. RightToLeft or RegexOptions. Some of these restrictions are fairly fundamental to the implementation, while some of them could be relaxed in time should there be sufficient demand.
NonBacktracking also has a subtle difference with regards to execution. If a capture group is in a loop, most engines only provide the last matched value for that capture, but.
As of now, the new RegexOptions. NonBacktracking only supports providing the last, as do most other regex implementations. For example, this code:. Beyond that, most anything you do today with Regex you can do with RegexOptions. Note that the goal of NonBacktracking is not to be always faster than the backtracking engines. In fact, one of the reasons backtracking engines are so popular is they can be extremely fast in the best and even expected cases, and the.
NET backtracking engines have been optimized with even more tricks and vectorization in. The backtracking engine implements that essentially by doing an IndexOf 'a' to find the first place to try to match.
Singleline matches everything , then LastIndexOf 'b' , and will declare success. In contrast, the non-backtracking engine will read a character in the input, look in a transition table to determine the next node to transition to, move to that node, and will rinse and repeat until it finds a match. The impact of that is evident in the resulting benchmark numbers:.
The strategy employed by the non-backtracking engine will be exactly the same: read a character, transition to the next node, read a character, transition to the next node, and so on. But the backtracking engine will end up having to do much more work. And again. And importantly, the time the non-backtracking engine took is almost exactly the same with both inputs.
Which is the whole point. For developers using Regex , Visual Studio has a really nice feature that provides syntax colorization, syntax validation, and regex IntelliSense when working with regular expressions. Historically, Visual Studio contained a hardcoded list of methods where it knew the arguments to those methods would be regular expressions.
NET 7 introduces the new [StringSyntax Now, any method that wants to indicate a string parameter accepts a regular expression can attribute it, e. Regex ] string expression , and Visual Studio will provide the same syntax validation, syntax coloring, and IntelliSense that it provides for all the other Regex -related methods. For example, the WebProxy class provides a constructor that accepts an array of regex strings to be used as proxy bypasses; this string[] parameter is attributed in.
String parameters, properties, and fields throughout the core. For example, you might write the pattern [a-z] in order to match an ASCII letter or digit, but you also want the uppercase values to be included. To achieve that, most modern regex engines have support for the? IgnoreCase option, which is equivalent to applying?
NET has also supported the RegexOptions. InvariantCulture option, which is only relevant when RegexOptions. IgnoreCase or? In every version of. NET prior to. NET 7, this case-insensivity support is implemented via ToLower. This support is functional, but there are some significant downsides to this implementation approach. ToLower overhead. ToLower isn't super expensive, but it's also not free. Having to call ToLower on every character in order to process it means a comparatively high cost to processing each value.
This overhead was decreased in previous versions of. NET, for example changing the code generated by RegexOptions. Compiled to cache the culture information so that rather than emitting the equivalent of CultureInfo. ToLower c. But even with such optimizations, this still contributes meaningfully to the gap in performance between case-sensitive and case-insensitive matching. Compiled RegexOptions.
But in the first case, with the set, in. With the former, the implementation in. NET 6 could use IndexOfAny 'A', 'a' to find the next possible start of a match, but because the case-insensitive implementation for IgnoreCase needs to call ToLower on every character, that implementation is forced to walk character by character through the input rather than vectorizing to process it in batches.
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