Which function is not valid in Terraform's string functions?

The function that is not valid in Terraform's string functions is Slice. In Terraform, string functions fulfill specific roles related to manipulating strings, but Slice is not among them.

The Join function is used to concatenate elements of a list into a single string, using a specified delimiter. Replace allows for specific substrings within a string to be replaced with another substring. The Split function divides a string into a list of substrings based on a specified delimiter. Each of these functions is part of Terraform's core capabilities for string manipulation.

On the other hand, Slice refers more to a collection of elements, such as lists or maps, and is used with data types that support indexing rather than direct string manipulation. This distinction highlights why Slice does not fit into the category of string functions within Terraform. Understanding the specific roles and applications of these functions is crucial for effective Terraform usage.