Zero-Width Assertions « Quantifiers « Regular Expressions

You can place place conditions on what should occur before or after a match, through lookbehind, lookahead, anchors, and word boundaries.

Expression	Meaning
^	Start of string (or line in multiline mode)
$	End of string (or line in multiline mode)
\A	Start of string (ignores multiline mode)
\z	End of string (ignores multiline mode)
\Z	End of line or string
\G	Where search started
\b	On a word boundary
\B	Not on a word boundary
(?=expr)	Continue matching only if expression expr matches on right (positive lookahead)
(?!expr)	Continue matching only if expression expr doesn't match on right (negative lookahead)
(?<=expr)	Continue matching only if expression expr matches on left (positive lookbehind)
(?<!expr)	Continue matching only if expression expr doesn't match on left (negative lookbehind)
(?>expr)	Subexpression expr is matched once and not backtracked

Lookahead and Lookbehind

The (?=expr) construct checks whether the text that follows matches expr, without including expr in the result.


using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
       Console.WriteLine (Regex.Match ("25 miles", @"\d+\s(?=miles)")); 
    }
}

The word "miles" was not returned in the result, even though it was required to satisfy the match.


using System;
using System.Text.RegularExpressions;


class Program
{
    static void Main(string[] args)
    {
       Console.WriteLine (Regex.Match ("25 miles more", @"\d+\s(?=miles).*")); 
    }
}


25 miles more

Use Lookahead to validate a strong password.

Suppose a password has to be at least six characters and contain at least one digit.


using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
         string password = "..."; 
         bool ok = Regex.IsMatch (password, @"(?=.*\d).{6,}"); 
    }
}


using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {

          string regex = "(?i)C#(?!.*(XML|Java))"; 
          Console.WriteLine (Regex.IsMatch ("C#! Java...",              regex));    
          Console.WriteLine (Regex.IsMatch ("C#! SQL!", regex));                
    }
}


False
True

(?<=expr) denotes positive lookbehind and requires that a match be preceded by a specified expression.

(?<!expr), denotes negative lookbehind and requires that a match not be preceded by a specified expression.

 
using System;
using System.Text.RegularExpressions;

class Program
{
    static void Main(string[] args)
    {
        string regex = "(?i)(?<!Java.*)Sql"; 
        Console.WriteLine (Regex.IsMatch ("Java, Sql...", regex)); 
        Console.WriteLine (Regex.IsMatch ("Java C#!", regex));     
    }
}


False
False