In Python, a string is a sequence of characters enclosed in single (') or double (") quotes. Strings are used to represent text data, such as names, messages, and code.
**Valid Strings**
A valid string in Python must meet the following criteria:
* It must be enclosed in quotes (either single or double).
* It can contain any Unicode character except the quote character used to enclose it (e.g., single quotes cannot be used inside a string enclosed in single quotes).
* It cannot contain any invalid characters, such as the "@" symbol.
**Invalid Strings**
The following strings are invalid in Python:
* Strings that are not enclosed in quotes
* Strings that contain invalid characters (e.g., "@")
* Strings that contain both single and double quotes (e.g., "'Hello, "world"!'")
**isValidString() Function**
The `isValidString()` function is a custom function that can be used to check if a given string is valid. It takes a string as input and returns True if the string is valid and False if it is invalid.
**How to Use isValidString()**
To use the `isValidString()` function, simply pass the string you want to check as the argument to the function. The function will return True if the string is valid and False if it is invalid.
For example:
```python
result = isValidString("Hello, world!") # True
result = isValidString("<a href="/coderai">@coderai</a>") # False
```