Extract substrings from a string
This article covers some different techniques for extracting parts of a string.
- Use the Split method when the substrings you want are separated by a known delimiting character (or characters).
- Regular expressions are useful when the string conforms to a fixed pattern.
- Use the IndexOf and Substring methods in conjunction when you don't want to extract all of the substrings in a string.
String.Split method
String.Split provides a handful of overloads to help you break up a string into a group of substrings based on one or more delimiting characters that you specify. You can choose to limit the total number of substrings in the final result, trim white-space characters from substrings, or exclude empty substrings.
The following examples show three different overloads of String.Split(). The first example calls the Split(Char[]) overload without passing any separator characters. When you don't specify any delimiting characters, String.Split() uses default delimiters, which are white-space characters, to split up the string.
string s = "You win some. You lose some.";
string[] subs = s.Split();
foreach (string sub in subs)
{
Console.WriteLine($"Substring: {sub}");
}
// This example produces the following output:
//
// Substring: You
// Substring: win
// Substring: some.
// Substring: You
// Substring: lose
// Substring: some.
Dim s As String = "You win some. You lose some."
Dim subs As String() = s.Split()
For Each substring As String In subs
Console.WriteLine("Substring: {0}", substring)
Next
' This example produces the following output:
'
' Substring: You
' Substring: win
' Substring: some.
' Substring: You
' Substring: lose
' Substring: some.
As you can see, the period characters (.) are included in two of the substrings. If you want to exclude the period characters, you can add the period character as an additional delimiting character. The next example shows how to do this.
string s = "You win some. You lose some.";
string[] subs = s.Split(' ', '.');
foreach (string sub in subs)
{
Console.WriteLine($"Substring: {sub}");
}
// This example produces the following output:
//
// Substring: You
// Substring: win
// Substring: some
// Substring:
// Substring: You
// Substring: lose
// Substring: some
// Substring:
Dim s As String = "You win some. You lose some."
Dim subs As String() = s.Split(" "c, "."c)
For Each substring As String In subs
Console.WriteLine("Substring: {0}", substring)
Next
' This example produces the following output:
'
' Substring: You
' Substring: win
' Substring: some
' Substring:
' Substring: You
' Substring: lose
' Substring: some
' Substring:
The periods are gone from the substrings, but now two extra empty substrings have been included. These empty substring represent the substring between the word and the period that follows it. To omit empty substrings from the resulting array, you can call the
Split(Char[], StringSplitOptions) overload and specify
StringSplitOptions.RemoveEmptyEntries for the options parameter.
string s = "You win some. You lose some.";
char[] separators = new char[] { ' ', '.' };
string[] subs = s.Split(separators, StringSplitOptions.RemoveEmptyEntries);
foreach (string sub in subs)
{
Console.WriteLine($"Substring: {sub}");
}
// This example produces the following output:
//
// Substring: You
// Substring: win
// Substring: some
// Substring: You
// Substring: lose
// Substring: some
Dim s As String = "You win some. You lose some."
Dim separators As Char() = New Char() {" "c, "."c}
Dim subs As String() = s.Split(separators, StringSplitOptions.RemoveEmptyEntries)
For Each substring As String In subs
Console.WriteLine("Substring: {0}", substring)
Next
' This example produces the following output:
'
' Substring: You
' Substring: win
' Substring: some
' Substring: You
' Substring: lose
' Substring: some
Regular expressions
If your string conforms to a fixed pattern, you can use a regular expression to extract and handle its elements. For example, if strings take the form "number operand number", you can use a regular expression to extract and handle the string's elements. Here's an example:
String[] expressions = { "16 + 21", "31 * 3", "28 / 3",
"42 - 18", "12 * 7",
"2, 4, 6, 8" };
String pattern = @"(\d+)\s+([-+*/])\s+(\d+)";
foreach (string expression in expressions)
{
foreach (System.Text.RegularExpressions.Match m in
System.Text.RegularExpressions.Regex.Matches(expression, pattern))
{
int value1 = Int32.Parse(m.Groups[1].Value);
int value2 = Int32.Parse(m.Groups[3].Value);
switch (m.Groups[2].Value)
{
case "+":
Console.WriteLine("{0} = {1}", m.Value, value1 + value2);
break;
case "-":
Console.WriteLine("{0} = {1}", m.Value, value1 - value2);
break;
case "*":
Console.WriteLine("{0} = {1}", m.Value, value1 * value2);
break;
case "/":
Console.WriteLine("{0} = {1:N2}", m.Value, value1 / value2);
break;
}
}
}
// The example displays the following output:
// 16 + 21 = 37
// 31 * 3 = 93
// 28 / 3 = 9.33
// 42 - 18 = 24
// 12 * 7 = 84
Dim expressions() As String = {"16 + 21", "31 * 3", "28 / 3",
"42 - 18", "12 * 7",
"2, 4, 6, 8"}
Dim pattern As String = "(\d+)\s+([-+*/])\s+(\d+)"
For Each expression In expressions
For Each m As Match In Regex.Matches(expression, pattern)
Dim value1 As Integer = Int32.Parse(m.Groups(1).Value)
Dim value2 As Integer = Int32.Parse(m.Groups(3).Value)
Select Case m.Groups(2).Value
Case "+"
Console.WriteLine("{0} = {1}", m.Value, value1 + value2)
Case "-"
Console.WriteLine("{0} = {1}", m.Value, value1 - value2)
Case "*"
Console.WriteLine("{0} = {1}", m.Value, value1 * value2)
Case "/"
Console.WriteLine("{0} = {1:N2}", m.Value, value1 / value2)
End Select
Next
Next
' The example displays the following output:
' 16 + 21 = 37
' 31 * 3 = 93
' 28 / 3 = 9.33
' 42 - 18 = 24
' 12 * 7 = 84
The regular expression pattern (\d+)\s+([-+*/])\s+(\d+) is defined like this:
| Pattern | Description |
|---|---|
(\d+) |
Match one or more decimal digits. This is the first capturing group. |
\s+ |
Match one or more white-space characters. |
([-+*/]) |
Match an arithmetic operator sign (+, -, *, or /). This is the second capturing group. |
\s+ |
Match one or more white-space characters. |
(\d+) |
Match one or more decimal digits. This is the third capturing group. |
You can also use a regular expression to extract substrings from a string based on a pattern rather than a fixed set of characters. This is a common scenario when either of these conditions occurs:
One or more of the delimiter characters does not always serve as a delimiter in the String instance.
The sequence and number of delimiter characters is variable or unknown.
For example, the Split method cannot be used to split the following string, because the number of \n (newline) characters is variable, and they don't always serve as delimiters.
[This is captured\ntext.]\n\n[\n[This is more captured text.]\n]
\n[Some more captured text:\n Option1\n Option2][Terse text.]
A regular expression can split this string easily, as the following example shows.
String input = "[This is captured\ntext.]\n\n[\n" +
"[This is more captured text.]\n]\n" +
"[Some more captured text:\n Option1" +
"\n Option2][Terse text.]";
String pattern = @"\[([^\[\]]+)\]";
int ctr = 0;
foreach (System.Text.RegularExpressions.Match m in
System.Text.RegularExpressions.Regex.Matches(input, pattern))
{
Console.WriteLine("{0}: {1}", ++ctr, m.Groups[1].Value);
}
// The example displays the following output:
// 1: This is captured
// text.
// 2: This is more captured text.
// 3: Some more captured text:
// Option1
// Option2
// 4: Terse text.
Dim input As String = String.Format("[This is captured{0}text.]" +
"{0}{0}[{0}[This is more " +
"captured text.]{0}{0}" +
"[Some more captured text:" +
"{0} Option1" +
"{0} Option2][Terse text.]",
vbCrLf)
Dim pattern As String = "\[([^\[\]]+)\]"
Dim ctr As Integer = 0
For Each m As Match In Regex.Matches(input, pattern)
ctr += 1
Console.WriteLine("{0}: {1}", ctr, m.Groups(1).Value)
Next
' The example displays the following output:
' 1: This is captured
' text.
' 2: This is more captured text.
' 3: Some more captured text:
' Option1
' Option2
' 4: Terse text.
The regular expression pattern \[([^\[\]]+)\] is defined like this:
| Pattern | Description |
|---|---|
\[ |
Match an opening bracket. |
([^\[\]]+) |
Match any character that is not an opening or a closing bracket one or more times. This is the first capturing group. |
\] |
Match a closing bracket. |
The Regex.Split method is almost identical to String.Split, except that it splits a string based on a regular expression pattern instead of a fixed character set. For example, the following example uses the Regex.Split method to split a string that contains substrings delimited by various combinations of hyphens and other characters.
String input = "abacus -- alabaster - * - atrium -+- " +
"any -*- actual - + - armoire - - alarm";
String pattern = @"\s-\s?[+*]?\s?-\s";
String[] elements = System.Text.RegularExpressions.Regex.Split(input, pattern);
foreach (string element in elements)
Console.WriteLine(element);
// The example displays the following output:
// abacus
// alabaster
// atrium
// any
// actual
// armoire
// alarm
Dim input As String = "abacus -- alabaster - * - atrium -+- " +
"any -*- actual - + - armoire - - alarm"
Dim pattern As String = "\s-\s?[+*]?\s?-\s"
Dim elements() As String = Regex.Split(input, pattern)
For Each element In elements
Console.WriteLine(element)
Next
' The example displays the following output:
' abacus
' alabaster
' atrium
' any
' actual
' armoire
' alarm
The regular expression pattern \s-\s?[+*]?\s?-\s is defined like this:
| Pattern | Description |
|---|---|
\s- |
Match a white-space character followed by a hyphen. |
\s? |
Match zero or one white-space character. |
[+*]? |
Match zero or one occurrence of either the + or * character. |
\s? |
Match zero or one white-space character. |
-\s |
Match a hyphen followed by a white-space character. |
String.IndexOf and String.Substring methods
If you aren't interested in all of the substrings in a string, you might prefer to work with one of the string comparison methods that returns the index at which the match begins. You can then call the Substring method to extract the substring that you want. The string comparison methods include:
IndexOf, which returns the zero-based index of the first occurrence of a character or string in a string instance.
IndexOfAny, which returns the zero-based index in the current string instance of the first occurrence of any character in a character array.
LastIndexOf, which returns the zero-based index of the last occurrence of a character or string in a string instance.
LastIndexOfAny, which returns a zero-based index in the current string instance of the last occurrence of any character in a character array.
The following example uses the IndexOf method to find the periods in a string. It then uses the Substring method to return full sentences.
String s = "This is the first sentence in a string. " +
"More sentences will follow. For example, " +
"this is the third sentence. This is the " +
"fourth. And this is the fifth and final " +
"sentence.";
var sentences = new List<String>();
int start = 0;
int position;
// Extract sentences from the string.
do
{
position = s.IndexOf('.', start);
if (position >= 0)
{
sentences.Add(s.Substring(start, position - start + 1).Trim());
start = position + 1;
}
} while (position > 0);
// Display the sentences.
foreach (var sentence in sentences)
Console.WriteLine(sentence);
// The example displays the following output:
// This is the first sentence in a string.
// More sentences will follow.
// For example, this is the third sentence.
// This is the fourth.
// And this is the fifth and final sentence.
Dim input As String = "This is the first sentence in a string. " +
"More sentences will follow. For example, " +
"this is the third sentence. This is the " +
"fourth. And this is the fifth and final " +
"sentence."
Dim sentences As New List(Of String)
Dim start As Integer = 0
Dim position As Integer
' Extract sentences from the string.
Do
position = input.IndexOf("."c, start)
If position >= 0 Then
sentences.Add(input.Substring(start, position - start + 1).Trim())
start = position + 1
End If
Loop While position > 0
' Display the sentences.
For Each sentence In sentences
Console.WriteLine(sentence)
Next
End Sub
' The example displays the following output:
' This is the first sentence in a string.
' More sentences will follow.
' For example, this is the third sentence.
' This is the fourth.
' And this is the fifth and final sentence.
