How to search strings
You can use two main strategies to search for text in strings. Methods of the String class search for specific text. Regular expressions search for patterns in text.
Note
The C# examples in this article run in the Try.NET inline code runner and playground. Select the Run button to run an example in an interactive window. Once you execute the code, you can modify it and run the modified code by selecting Run again. The modified code either runs in the interactive window or, if compilation fails, the interactive window displays all C# compiler error messages.
The string type, which is an alias for the System.String class, provides a number of useful methods for searching the contents of a string. Among them are Contains, StartsWith, EndsWith, IndexOf, LastIndexOf. The System.Text.RegularExpressions.Regex class provides a rich vocabulary to search for patterns in text. In this article, you learn these techniques and how to choose the best method for your needs.
Does a string contain text?
The String.Contains, String.StartsWith, and String.EndsWith methods search a string for specific text. The following example shows each of these methods and a variation that uses a case-insensitive search:
string factMessage = "Extension methods have all the capabilities of regular static methods.";
// Write the string and include the quotation marks.
Console.WriteLine($"\"{factMessage}\"");
// Simple comparisons are always case sensitive!
bool containsSearchResult = factMessage.Contains("extension");
Console.WriteLine($"Contains \"extension\"? {containsSearchResult}");
// For user input and strings that will be displayed to the end user,
// use the StringComparison parameter on methods that have it to specify how to match strings.
bool ignoreCaseSearchResult = factMessage.StartsWith("extension", System.StringComparison.CurrentCultureIgnoreCase);
Console.WriteLine($"Starts with \"extension\"? {ignoreCaseSearchResult} (ignoring case)");
bool endsWithSearchResult = factMessage.EndsWith(".", System.StringComparison.CurrentCultureIgnoreCase);
Console.WriteLine($"Ends with '.'? {endsWithSearchResult}");
The preceding example demonstrates an important point for using these methods. Searches are case-sensitive by default. You use the StringComparison.CurrentCultureIgnoreCase enumeration value to specify a case-insensitive search.
Where does the sought text occur in a string?
The IndexOf and LastIndexOf methods also search for text in strings. These methods return the location of the text being sought. If the text isn't found, they return -1. The following example shows a search for the first and last occurrence of the word "methods" and displays the text in between.
string factMessage = "Extension methods have all the capabilities of regular static methods.";
// Write the string and include the quotation marks.
Console.WriteLine($"\"{factMessage}\"");
// This search returns the substring between two strings, so
// the first index is moved to the character just after the first string.
int first = factMessage.IndexOf("methods") + "methods".Length;
int last = factMessage.LastIndexOf("methods");
string str2 = factMessage.Substring(first, last - first);
Console.WriteLine($"Substring between \"methods\" and \"methods\": '{str2}'");
Finding specific text using regular expressions
The System.Text.RegularExpressions.Regex class can be used to search strings. These searches can range in complexity from simple to complicated text patterns.
The following code example searches for the word "the" or "their" in a sentence, ignoring case. The static method Regex.IsMatch performs the search. You give it the string to search and a search pattern. In this case, a third argument specifies case-insensitive search. For more information, see System.Text.RegularExpressions.RegexOptions.
The search pattern describes the text you search for. The following table describes each element of the search pattern. (The table below uses the single \, which must be escaped as \\ in a C# string).
| Pattern | Meaning |
|---|---|
the |
match the text "the" |
(eir)? |
match 0 or 1 occurrence of "eir" |
\s |
match a white-space character |
string[] sentences =
{
"Put the water over there.",
"They're quite thirsty.",
"Their water bottles broke."
};
string sPattern = "the(ir)?\\s";
foreach (string s in sentences)
{
Console.Write($"{s,24}");
if (System.Text.RegularExpressions.Regex.IsMatch(s, sPattern, System.Text.RegularExpressions.RegexOptions.IgnoreCase))
{
Console.WriteLine($" (match for '{sPattern}' found)");
}
else
{
Console.WriteLine();
}
}
Tip
The string methods are usually better choices when you are searching for an exact string. Regular expressions are better when you are searching for some pattern in a source string.
Does a string follow a pattern?
The following code uses regular expressions to validate the format of each string in an array. The validation requires that each string have the form of a telephone number in which three groups of digits are separated by dashes, the first two groups contain three digits, and the third group contains four digits. The search pattern uses the regular expression ^\\d{3}-\\d{3}-\\d{4}$. For more information, see Regular Expression Language - Quick Reference.
| Pattern | Meaning |
|---|---|
^ |
matches the beginning of the string |
\d{3} |
matches exactly 3 digit characters |
- |
matches the '-' character |
\d{4} |
matches exactly 4 digit characters |
$ |
matches the end of the string |
string[] numbers =
{
"123-555-0190",
"444-234-22450",
"690-555-0178",
"146-893-232",
"146-555-0122",
"4007-555-0111",
"407-555-0111",
"407-2-5555",
"407-555-8974",
"407-2ab-5555",
"690-555-8148",
"146-893-232-"
};
string sPattern = "^\\d{3}-\\d{3}-\\d{4}$";
foreach (string s in numbers)
{
Console.Write($"{s,14}");
if (System.Text.RegularExpressions.Regex.IsMatch(s, sPattern))
{
Console.WriteLine(" - valid");
}
else
{
Console.WriteLine(" - invalid");
}
}
This single search pattern matches many valid strings. Regular expressions are better to search for or validate against a pattern, rather than a single text string.
