Input: A text string, using prompt.

Output: Either "Valid name" or "Invalid name", depending on whether the input names fit the required format, which is
Last name,first name, middle initial
where neither of the names have more than 15 characters.

Well, aren't you specific with your name requirements! Let's see if your names pass the test, shall we?

```
def validate_name(name):
names = name.split(",")
if len(names) != 3:
return "Invalid name"

last_name = names[0].strip()
first_name = names[1].strip()
middle_initial = names[2].strip()

if len(last_name) > 15 or len(first_name) > 15 or len(middle_initial) > 1:
return "Invalid name"

return "Valid name"
```

Using this function, you can validate your name strings. However, keep in mind that names can be quite clownish sometimes, so if you get an "Invalid name" response, you might want to double-check to make sure you're not dealing with a flamboyant jester.

To check if a given name fits the required format, which is "Last name, first name, middle initial," without any name exceeding 15 characters, follow these step-by-step instructions:

1. Start by splitting the given text string into three separate names using the comma (',') as a delimiter. One name will represent the last name, the second name will represent the first name, and the third name will represent the middle initial.

2. Check if each name (last name, first name, and middle initial) has a length greater than 0 and less than or equal to 15 characters. If any name exceeds 15 characters or is empty, return "Invalid name."

3. After passing the length check, validate that each name only contains alphabetical characters. Ensure that each name consists only of letters from A to Z or a to z. If any name contains other characters, return "Invalid name."

4. If all three names pass the length and character validation, return "Valid name," as per the required format.

Here is the code implementation in Python:

```python
def validate_names(name_string):
names = name_string.split(',')

if len(names) != 3:
return "Invalid name"

last_name = names[0].strip()
first_name = names[1].strip()
middle_initial = names[2].strip()

if len(last_name) > 15 or len(first_name) > 15 or len(middle_initial) > 15:
return "Invalid name"

if not last_name.isalpha() or not first_name.isalpha() or not middle_initial.isalpha():
return "Invalid name"

return "Valid name"
```

You can call the `validate_names` function with a name string as an argument to determine if it is a valid name or an invalid name. For example:

```python
name_input = "Doe,John,D."
result = validate_names(name_input)
print(result) # "Valid name"
```

Hope this helps! Let me know if you have any further questions.

To determine whether a given name fits the required format, we need to check if it consists of three parts: last name, first name, and middle initial. Additionally, none of these names should have more than 15 characters.

Here's how we can check the input name:

1. Prompt the user to enter the name in the required format: "Last name, first name, middle initial".
2. Split the input string using the comma as a delimiter to separate the three parts: last name, first name, and middle initial. You can use the split() method in most programming languages.
3. Check if there are exactly three parts after splitting. If not, the name is invalid, and you can output "Invalid name" immediately.
4. For each part, check if its length exceeds 15 characters. If any part is longer than 15 characters, the name is invalid, and you can output "Invalid name".
5. If all three parts pass the length check and there are exactly three parts, you can output "Valid name".

Here's a Python implementation that demonstrates the steps above:

```python
def check_name(input_name):
name_parts = input_name.split(",")

if len(name_parts) != 3:
return "Invalid name"

for part in name_parts:
if len(part.strip()) > 15:
return "Invalid name"

return "Valid name"

# Example usage
input_name = input("Enter the name in the format 'Last name, first name, middle initial': ")
result = check_name(input_name)
print(result)
```

You can adapt this logic to the programming language of your choice, as the steps involved are fairly simple.