.NET Regex Tutorial

posted in dotnet on 19 April 2019 • by Wouter Van Schandevijl

Not nearly as confusing as it is in JavaScript.

using System.Text.RegularExpressions;

bool mach = Regex.IsMatch("input", @"\w+", RegexOptions.None);
Match match = Regex.Match("input", @"\w+");
IReadOnlyList<Match> matches = Regex.Matches("input", @"\w+");
string result = Regex.Replace("input", @"(\w+)", "$1");

When executing the same regex many times, prefer the static methods on Regex over new Regex() as they keep an internal cache for performance.

Its methods may throw a RegexMatchTimeoutException

Creation

var regex = new Regex(@"\d+", RegexOptions.None);

var commentedRegex = new Regex(@"
    \d    # Any digit
    [0-9] # With character class
    ", RegexOptions.IgnorePatternWhitespace);

RegexOptions.Compiled: Increased startup time for decreased execution time.
RegexOptions.IgnoreCase.
RegexOptions.ExplicitCapture: Only explicitly named/numbered groups are captured. (Skip the (?:…)).
RegexOptions.Multiline: ^ and $ match beginning and end of line.
RegexOptions.Singleline: . matches \n.

Tes A Match

Match match = Regex.Match("1-22-333", @"(\d+)-(\d+)-(\d+)(-\d+)?");

IReadOnlyList<Group> groups = match.Groups;
Assert.Equal(5, groups.Count);

Assert.Equal("1-22-333", groups.First().Value);

Group one = match.Groups.Skip(1).First();
Assert.Equal("1", one.Value);

Group fours = match.Groups.Last();
Assert.False(fours.Success);

Multiple Named Matches

string pattern = @"(?<one>\d+)-(?<two>\d+)";
IReadOnlyList<Match> matches = Regex.Matches("1-22, 1-2", pattern);

Assert.Equal(2, matches.Count);

Match first = matches.First();
Assert.Equal("1-22", first.Value);

Match second = matches.Last();
Assert.Equal("1-2", second.Value);


IReadOnlyList<Group> groups = first.Groups;
Group one = groups.Skip(1).First();
Assert.Equal("one", one.Name);
Assert.Equal("1", one.Value);

Replace

Basic Usage

var regex = new Regex(@"(\d)-(\d+)");
var result = regex.Replace("1-22", "$1+$2");
Assert.Equal("1+22", result);

result = Regex.Replace("1", @"(?<amount>\d+)", "$$ ${amount}");
Assert.Equal("$ 1", result);

Replace with Function

string input = "Hello World!";
string pattern = @"Hello";

var result = Regex.Replace(input, pattern, (Match m) => $"<b>{m.Value}</b>");
Assert.Equal("<b>Hello</b> World!", result);

Other Substitutions

$& : entire match
$` : before
$' : after
$+ : last captured group
$_ : entire input string