The efficiency of a 'CASE' structure in terms of computational resources generally depends on the language implementation and the specific scenario. In many programming languages, a 'CASE' structure can be more efficient than a series of 'IF-ELSE' statements, particularly when dealing with a large number of conditions.

In a 'CASE' structure, the variable or expression is evaluated only once, and then the control is directly transferred to the matching case. This can lead to reduced evaluation time, especially when the number of conditions is high. In contrast, an 'IF-ELSE IF' ladder evaluates each condition in turn until it finds a match, which can be less efficient, especially if the matching case is towards the end of the ladder.

However, the actual performance difference in modern programming environments may be negligible for most practical purposes. Compilers and interpreters often optimize both 'CASE' structures and 'IF-ELSE' statements effectively. The choice between these structures should therefore be based more on readability, maintainability, and the specific requirements of the problem rather than on performance alone.

A 'CASE' structure can indeed be used for string manipulation and pattern matching, though its capabilities might be somewhat limited compared to regular expressions or dedicated string manipulation functions. In programming languages that support string evaluation in switch-case constructs, the 'CASE' structure can be used to match specific strings or patterns.

For instance, in a scenario where a program needs to respond to specific commands input as strings (like 'start', 'stop', 'pause'), a 'CASE' structure can efficiently handle this. Each case would match a particular command string, and the corresponding block of code would execute the desired action.

However, it's important to note that while a 'CASE' structure can handle exact string matches, it's not as flexible as regular expressions for complex pattern matching. It's ideal for scenarios where you have a known set of string values to compare against. For more complex string manipulations or when dealing with patterns that can have variations, regular expressions or specific string handling functions are more appropriate and powerful.

The 'CASE' structure and an 'IF-ELSE IF' ladder both serve to execute different blocks of code based on certain conditions, but they differ in syntax and use-case scenarios. A 'CASE' structure is typically used when a single variable or expression is being checked against multiple discrete values. It provides a clearer and more organised way to handle such scenarios, especially when the number of conditions is high. Each 'CASE' within the structure corresponds to a specific value, making the code more readable and easier to manage.

On the other hand, an 'IF-ELSE IF' ladder is more versatile and is used for checking a series of conditions that might not be just discrete values. It's more flexible in terms of the types of conditions it can handle, including ranges, comparisons, and complex boolean logic. This structure is preferable when the conditions are not just based on different values of a single variable but involve different expressions or a combination of variables.

In summary, use a 'CASE' structure for clarity and organisation when dealing with multiple discrete conditions based on a single variable. Opt for an 'IF-ELSE IF' ladder when dealing with more complex, varied conditions that go beyond simple value checking.

Nesting a 'CASE' structure within another 'CASE' structure involves placing one switch-case construct inside another. This is particularly useful when decision-making processes are hierarchical or when a condition depends on the outcome of another condition. The outer 'CASE' structure evaluates the first level of conditions, and based on the outcome, the control moves to the respective inner 'CASE' structure, which then evaluates its set of conditions.

For example, consider a scenario in a school system where you first need to determine the grade level of a student (like primary, secondary, or higher education) and then apply different criteria or actions based on subjects within each grade level. The outer 'CASE' would differentiate between the grade levels, and the inner 'CASE' structures would handle subject-specific actions or criteria.

Nesting 'CASE' structures can greatly enhance the readability and structure of the code, especially in complex decision-making scenarios. It allows for a cleaner and more hierarchical approach to handling multi-level conditions, making the code easier to understand and maintain. However, it's essential to ensure that the code doesn't become overly complicated or deeply nested, as this can lead to the opposite effect, making it hard to follow and debug.

The possibility of using variables or expressions as case labels in a 'CASE' structure largely depends on the programming language's capabilities. Some languages strictly require case labels to be constant expressions, while others may allow variables or even more complex expressions.

In languages that support variable or expression-based case labels, this feature can significantly enhance the flexibility of the 'CASE' structure. It allows the structure to adapt to dynamic conditions, where the cases are not predetermined but rather depend on the program's state or user input. For example, a 'CASE' structure could be used to handle different scenarios based on user-defined settings or real-time data.

However, this flexibility comes with its considerations. When using variables or expressions as case labels, it's crucial to ensure that these values are well-defined and do not lead to ambiguity or unpredictable behaviour. The code must be carefully designed to handle all possible outcomes and to maintain clarity and predictability. Additionally, this approach may introduce complexity, making the code harder to read and understand, especially for those who are accustomed to traditional, constant-expression case labels. Thus, while variable or expression-based case labels can increase the power of a 'CASE' structure, they should be used judiciously and with a clear understanding of the implications for the code's logic and readability.