strsubst

Replacing expressions within strings. An expression is defined as possibly nested
sequence of text, operators and subexpressions in braces.

This is similar to the string expansions many shells offer, with a slightly different syntax and semantic as the expressions are completely within braces and operators work on text, not variables.

In unix shell '${x}' expands a variable, here it is {$x} for example.

There are operators for many common tasks like defining variables, string handling like search/replacment, math and comparisons.