TheAlgorithms/C-Sharp · error · ArgumentException

Invalid character ' ' found in the expression.

Error message

Invalid character '{c}' found in the expression.

What it means

BalancedParenthesesChecker.IsBalanced throws ArgumentException when the expression contains a character that is neither an opening nor a closing parenthesis/bracket/brace. The library only recognizes known bracket symbols, so any other character makes the input undefined for the checker and it fails fast rather than silently ignoring it.

Solutions

1. Strip non-bracket characters from the input before calling IsBalanced, e.g. Regex.Replace(input, "[^()\[\]{}]", "").
2. Wrap the call in a try/catch for ArgumentException and treat the message as user-facing validation feedback.
3. If letters/operators are legitimate input, pre-validate or use an expression parser suited to that grammar instead of this checker.

Example fix

// before
bool ok = checker.IsBalanced("(a + b) * [c]");
// after
string bracketsOnly = Regex.Replace("(a + b) * [c]", "[^()\[\]{}]", "");
bool ok = checker.IsBalanced(bracketsOnly);

if (Regex.IsMatch(expression, "[^()\[\]{}]")) throw new ArgumentException("Expression must contain only ()[]{} characters.");

bool IsBracketOnly(string s) => !string.IsNullOrEmpty(s) && s.All(c => c is '(' or ')' or '[' or ']' or '{' or '}');

try { checker.IsBalanced(expr); } catch (ArgumentException ex) { /* show ex.Message as input validation error */ }

When it happens

Trigger: Calling IsBalanced with an expression containing characters outside the recognized bracket set, e.g. IsBalanced("(a[b]{c})") or IsBalanced("<>").

Common situations: Users pass raw source-code snippets, math expressions with operators/letters, or XML-style angle brackets instead of a pure bracket-only string.

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.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• Invalid character .
• message
• key must be non-empty string
• k must be at least 1.
• Input features cannot be empty.

AI-assisted analysis of TheAlgorithms/C-Sharp@96e2905cab (2026-09-13). Data as JSON: /api/errors/021dd4b991e8744e. Report an issue: GitHub.

Appendix: source

Thrown at Algorithms/Stack/BalancedParenthesesChecker.cs:54

        }

        Stack<char> stack = new Stack<char>();
        foreach (char c in expression)
        {
            if (IsOpeningParenthesis(c))
            {
                stack.Push(c);
            }
            else if (IsClosingParenthesis(c))
            {
                if (!IsBalancedClosing(stack, c))
                {
                    return false;
                }
            }
            else
            {
                throw new ArgumentException($"Invalid character '{c}' found in the expression.");
            }
        }

        return stack.Count == 0;
    }

    private static bool IsOpeningParenthesis(char c)
    {
        return c == '(' || c == '{' || c == '[';
    }

    private static bool IsClosingParenthesis(char c)
    {
        return c == ')' || c == '}' || c == ']';
    }

    private static bool IsBalancedClosing(Stack<char> stack, char close)
    {

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