The "format" operator is used to create formatted strings based on a given pattern. It takes one or more arguments, including a string that specifies the format pattern and other objects that are used to fill in the placeholders in the pattern.

The format pattern consists of two types of characters: plain characters and format specifiers. Plain characters are literal characters that appear in the format string as themselves and do not have any special meaning or function, while format specifiers are placeholders that start with a percent sign (%) and end with a conversion character that indicates the type of value to be inserted.

Example:

context Person

self.name.format('My name is %s and I am %d years old.', self.name, self.age) = self.introduction

In this example, we define an OCL constraint for a Person class, which requires that the introduction attribute is equal to a formatted string generated using the format operator. The format pattern is specified as the first argument to the format operator, and it contains two format specifiers: %s for the name (a string value) and %d for the age (an integer value).

The remaining arguments to the format operator are the values that should be substituted into the placeholders in the format pattern. In this case, we use self.name and self.age to fill in the placeholders.

If the name of the person is Alice and her age is 30, then the result of the format operator will be the string My name is Alice and I am 30 years old., which should be equal to the value of the introduction attribute of the person object. If this constraint is violated, it indicates that there is an error in the definition of the Person class.