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Asked: 2024-09-22T21:13:18+05:30 In:

How can I implement JavaScript validation to restrict input such that only a single uppercase letter is allowed in a given string?

Hey everyone! I’m working on a web project, and I’ve stumbled upon a challenge with JavaScript validation. I need to restrict input in a form field such that only a single uppercase letter is allowed in a string. For example, inputs like “A”, “hello A”, or “hello World” should be accepted, but “a”, “HELLO”, or “A B” should be rejected.

Does anyone have ideas on how I can implement this validation effectively? It would be great to see some code snippets or examples if you have them. Thanks in advance for your help!

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    2 Answers

    1. Certainly! You can use JavaScript to validate the input field. Below is an example of how you could structure your HTML form and include JavaScript validation to ensure only one uppercase letter is present in the input string:

      

      

      

      

      

      

      Form Validation
      

      

      function validateInput() {
      

      var input = document.getElementById("myInput").value;
      

      var regex = /^(?=.*[A-Z])(?!.*[A-Z]{2}).*$/;
      

      if (regex.test(input)) {
      

      alert("Valid input.");
      

      return true;
      

      } else {
      

      alert("Invalid input - ensure there is only a single uppercase letter in the string.");
      

      return false;
      

      }
      

      }
      

      

      

      

      

      

      

      

      

      

      In this code:
      

      – The HTML form (`
      

      `) contains an input field and a submit button.
      

      

      – The `validateInput` function is called on form submission.
      

      – The regular expression `/^(?=.*[A-Z])(?!.*[A-Z]{2}).*$/` checks for the presence of exactly one uppercase letter:
      

      –
      
							
      
							
      
														
      
														
      
							
      
								
															
      
						
      
						
      
					
      
				
      
							
		
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      To achieve the validation where only a single uppercase letter is allowed in a string, you can use a combination of a regular expression and JavaScript. Here’s a simple example using the onsubmit event of a form to validate the input field before it’s submitted:
      

      

      

      

      

      

      Single Uppercase Letter Validation
      

      

      function validateInput(input) {
      

      var regex = /^(.*?)[A-Z](.*?)$/;
      

      if (input.value.match(regex) && !input.value.match(/[A-Z].*?[A-Z]/)) {
      

      alert("Valid input!");
      

      return true;
      

      } else {
      

      alert("Invalid input. Only a single uppercase letter is allowed.");
      

      return false;
      

      }
      

      }
      

      

      

      

      

      

      

      

      

      

      In this example:
      

      
1. The `validateInput` function is called when the form is submitted.
      
2. The function uses the regex `/^(.*?)[A-Z](.*?)$/` to check if the input contains at least one uppercase letter.
      3. The `!input.value.match(/[A-Z].
      
							
      
							
      
														
      
														
      
							
      
								
															
      
						
      
						
      
					
      
				
      
							
		
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function validateInput(input) {
    const regex = /^(?=.*[A-Z])(?!.*[A-Z].*[A-Z])[A-Za-z\s]*$/;
    return regex.test(input);
}

// Example usage
const testInputs = ["A", "hello A", "hello World", "a", "HELLO", "A B"];
testInputs.forEach(input => {
    console.log(`${input}: ${validateInput(input)}`);
});
  
      


      

      
							
      
							
      
														
      
														
      
							
      
								
															
      
						
      
						
      
					
      
				
      
							
		
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    JavaScript Validation Example
      
    
      

      

      

      Validation of Uppercase Letters
      

      
        
      
        
      
        
      
    
      


      

      
							
      
							
      
														
      
														
      
							
      
								
															
      
						
      
						
      
					
      
				
      
							
		
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