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.
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.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Mar 22, 2023 (simplify).
