Mega Code Archive
0649 Use Lookahead to validate a strong password
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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,}");
}
}
(?!expr) is the negative lookahead construct.
This requires that the match not be followed by expr.
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));
}
}
The output:
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));
}
}
The output:
False
False
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