/**
 * Validation rule function type.
 * Returns error message string if invalid, undefined/null if valid.
 */
export type ValidationRule<T = unknown> = (value: T, formData: Record<string, unknown>) => string | undefined | null;
/**
 * Field validation rules configuration.
 */
export interface FieldRules<T = unknown> {
    required?: boolean | string;
    minLength?: number | {
        value: number;
        message: string;
    };
    maxLength?: number | {
        value: number;
        message: string;
    };
    min?: number | {
        value: number;
        message: string;
    };
    max?: number | {
        value: number;
        message: string;
    };
    pattern?: RegExp | {
        value: RegExp;
        message: string;
    };
    email?: boolean | string;
    custom?: ValidationRule<T> | ValidationRule<T>[];
}
export declare function validateFieldValue(value: unknown, fieldRules: FieldRules | undefined, formData: Record<string, unknown>): string | null;