C# - String Between, Before, After

Between, before, after. With extension methods, we easily locate Substrings. We search for substrings between two strings, before a string or after a string.

Method notes. In these extension methods, we return relative substrings. This simplifies certain programs. We implement some logic with IndexOf and LastIndexOf.

String Substring

String IndexOf

String LastIndexOf

Input and output. To begin, we show the input string, and our desired output when we call two arguments. Consider a simple language. We can parse a variable name this way (A).

Input = "DEFINE:A=TWO"

Between("DEFINE:", "=TWO") = "A"

An example. We see the SubstringExtensions static class. This contains the extension methods Between, Before and After. We call these methods in Main.

Extension

Detail 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.

Detail In main() 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.

Console.WriteLine

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);
    }
}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("="));
    }
}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 We could use a simple loop along with these extensions to parse a small language.

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.