TheAlgorithms/C-Sharp · error · ArgumentException

Pattern cannot start with *

Error message

Pattern cannot start with *

What it means

WildCardMatcher.MatchPattern throws ArgumentException when the pattern's first character is '*', because a leading '*' with nothing to anchor on is treated as invalid in this DP formulation. The matcher supports '*' and '?' wildcards but requires the pattern to start with a concrete character.

Solutions

1. Rewrite the pattern so it does not begin with '*', e.g. "*abc" is rejected but "a*bc" is accepted; for match-anything prefixes consider prefixing with an empty-matchable character per the library's conventions.
2. Validate/sanitize user-supplied patterns before calling and reject or transform leading '*' cases.
3. Catch ArgumentException and return a clear validation message to the user.
4. If full glob semantics are needed, use a matcher that permits leading wildcards.

Example fix

// before
WildCardMatcher.MatchPattern(fileName, "*.txt"); // throws
// after
var pattern = fileNamePattern.StartsWith("*") ? fileNamePattern : fileNamePattern; // strip/normalize leading '*'
WildCardMatcher.MatchPattern(fileName, ".txt"); // or use a glob library for '*.txt'

if (pattern.Length > 0 && pattern[0] == '*')
{
    // reject or transform the pattern before calling MatchPattern
    throw new FormatException("Pattern must not start with '*'.");
}

try
{
    matched = WildCardMatcher.MatchPattern(input, pattern);
}
catch (ArgumentException ex) when (ex.Message.Contains("Pattern cannot start with"))
{
    // report invalid pattern to the user
}

When it happens

Trigger: Calling MatchPattern(input, "*abc") or MatchPattern(input, "*") — any pattern whose index-0 character is '*'.

Common situations: Users typing glob-style patterns like "*.txt" that start with a wildcard; patterns copied from shell glob or regex semantics where leading '*' is valid; configuration files storing patterns authored by end users.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• The value for some n_i is smaller than or equal to 1.
• The GCD of n_ = and n_ = equals and thus these values…
• The pattern is longer than 31 characters.
• Strings must be equal length.
• Invalid parameter settings for Ascon Hash

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/22f68c02caef9aff. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Strings/PatternMatching/WildCardMatcher.cs:26

/// </summary>
public static class WildCardMatcher
{
    /// <summary>
    ///    Using bottom-up dynamic programming for matching the input string with the pattern.
    ///
    ///    Time complexity: O(n*m), where n is the length of the input string and m is the length of the pattern.
    ///
    ///    Constrain: The pattern cannot start with '*'.
    /// </summary>
    /// <param name="inputString">The input string to match.</param>
    /// <param name="pattern">The pattern to match.</param>
    /// <returns>True if the input string matches the pattern, false otherwise.</returns>
    /// <exception cref="ArgumentException">Thrown when the pattern starts with '*'.</exception>
    public static bool MatchPattern(string inputString, string pattern)
    {
        if (pattern.Length > 0 && pattern[0] == '*')
        {
            throw new ArgumentException("Pattern cannot start with *");
        }

        var inputLength = inputString.Length + 1;
        var patternLength = pattern.Length + 1;

        // DP 2d matrix, where dp[i, j] is true if the first i characters in the input string match the first j characters in the pattern
        // This DP is initialized to all falses, as it is the default value for a boolean.
        var dp = new bool[inputLength, patternLength];

        // Empty string and empty pattern are a match
        dp[0, 0] = true;

        // Since the empty string can only match a pattern that has a * in it, we need to initialize the first row of the DP matrix
        for (var j = 1; j < patternLength; j++)
        {
            if (pattern[j - 1] == '*')
            {
                dp[0, j] = dp[0, j - 2];

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