In the Object Constraint Language (OCL), the String data type represents sequences of characters. Here's an explanation of the String data type using OCL:

1. **Definition**: In OCL, a String is defined as a finite sequence of characters from some character set. It can include letters, digits, punctuation marks, and other symbols.

2. **Syntax**: The syntax for declaring a String variable in OCL is straightforward. For example:

  ```
  variableName: String
  ```
  This declares a variable named `variableName` of type String.

3. **Operations**: OCL provides several operations that can be performed on String data types. Some common operations include:

  - Concatenation: Combining two strings together to form a new string.
  - Length: Determining the number of characters in a string.
  - Substring: Extracting a portion of a string.
  - Comparison: Comparing two strings for equality or ordering.
  - Conversion: Converting a string to upper case, lower case, or other formats.

4. **Constraints**: In OCL, constraints can be applied to String data types to specify rules or conditions that must be satisfied. For example, constraints can specify the minimum or maximum length of a string, character patterns it must match, or any other relevant conditions.

5. **Examples**:

  - Concatenation:
    ```
    let str1: String = "Hello";
    let str2: String = " World";
    let result: String = str1.concat(str2);  -- Result will be "Hello World"
    ```

  - Length:
    ```
    let str: String = "Hello";
    let length: Integer = str.size();  -- Length will be 5
    ```

  - Substring:
    ```
    let str: String = "Hello World";
    let sub: String = str.substring(6, 5);  -- Substring will be "World"
    ```

  - Comparison:
    ```
    let str1: String = "apple";
    let str2: String = "banana";
    let isEqual: Boolean = str1 = str2;  -- isEqual will be false
    ```

  - Constraint:
    ```
    context Person
    inv: name.size() >= 2  -- Ensures that the name of a person must be at least 2 characters long
    ```

In summary, the String data type in OCL represents sequences of characters and provides operations and constraints for manipulating and constraining strings in model constraints and expressions.

See also: Number conversions