C# String Between, Before, After

Create Between, Before and After extension methods and use them to parse text.
Between, before, after. With extension methods, we easily locate Substrings. We search for substrings between two strings, before a string or after a string.
In these extension methods, we return relative substrings. This simplifies certain programs. We implement some logic with IndexOf and LastIndexOf.SubstringIndexOfLastIndexOf
An example. We see the SubstringExtensions static class. This contains the extension methods Between, Before and After. We call these methods in Main.

Between: The Between method accepts 2 strings (a and b) and locates each in the source string.

Then: After some error checking, it returns the substring between them. The Before method and After method are similar.

Main: Here an input string "test" is declared. The extensions Between, Before, and After are used on it.

And: The results are printed to the console (with Console.WriteLine) after the parts are extracted.

SubstringExtensions.cs implementation: C# static class SubstringExtensions { /// <summary> /// Get string value between [first] a and [last] b. /// </summary> public static string Between(this string value, string a, string b) { int posA = value.IndexOf(a); int posB = value.LastIndexOf(b); if (posA == -1) { return ""; } if (posB == -1) { return ""; } int adjustedPosA = posA + a.Length; if (adjustedPosA >= posB) { return ""; } return value.Substring(adjustedPosA, posB - adjustedPosA); } /// <summary> /// Get string value after [first] a. /// </summary> public static string Before(this string value, string a) { int posA = value.IndexOf(a); if (posA == -1) { return ""; } return value.Substring(0, posA); } /// <summary> /// Get string value after [last] a. /// </summary> public static string After(this string value, string a) { int posA = value.LastIndexOf(a); if (posA == -1) { return ""; } int adjustedPosA = posA + a.Length; if (adjustedPosA >= value.Length) { return ""; } return value.Substring(adjustedPosA); } } Main method: C# using System; class Program { static void Main() { // Input. const string test = "DEFINE:A=TWO"; // Test Between. Console.WriteLine(test.Between("DEFINE:", "=")); Console.WriteLine(test.Between(":", "=")); // Test Before. Console.WriteLine(test.Before(":")); Console.WriteLine(test.Before("=")); // Test After. Console.WriteLine(test.After(":")); Console.WriteLine(test.After("DEFINE:")); Console.WriteLine(test.After("=")); } } Output A A DEFINE DEFINE:A A=TWO A=TWO TWO
Use. Sometimes, a programmer decides a problem is best solved with a small language. However, often the requirements are not complex enough to warrant a full grammar and a parser engine.

So: You could use a simple loop along with these extensions to parse a small language.

Correctness. These methods are in use in a program I developed. There may be some errors in the error-handling logic in some cases—this has not been studied.
A summary. We provided the implementations for Between, Before, and After extensions on the string type. These implementations are not fully compatible with all requirements.
They only search for the first or last instance of the parameters. They are useful in simplifying parsing engines for simple custom languages.
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