String Template Expression Evaluator

Implement an interpreter for a domain-specific string macro and expression language. The language supports string literals, variable bindings with lexical scoping, and four built-in string manipulation functions.

Syntax & Rules

1. Whitespace: Spaces, tabs, and newlines between tokens are ignored (except inside quoted string literals).
2. String Literals: Enclosed in double quotes "...". They can contain escaped double quotes \" and escaped backslashes \\.
3. Keywords & Identifiers:
   • Keywords: let, in
   • Built-in functions: concat, repeat, slice, reverse
   • Variable names: Valid identifiers containing letters, digits, and underscores, starting with a letter or underscore. Variable names cannot match keywords or built-in function names.
4. Expressions (Expr):
   • String Literal: e.g., "hello"
   • Variable Reference: e.g., x. Unbound variable references evaluate to the empty string "".
   • Let Binding: let <ident> = <Expr1> in <Expr2> Evaluates <Expr1>, binds the resulting string to <ident> in a new lexical scope, and evaluates <Expr2>. Inner bindings shadow outer bindings of the same variable name.
   • Function Calls:
     • concat(Expr1, Expr2, ...): Evaluates all arguments in order and concatenates their resulting strings. Accepts 0 or more comma-separated arguments. concat() returns "".
     • repeat(Expr_str, Expr_count): Evaluates Expr_str to string $S$, and Expr_count to string $K_{str}$. Parses $K_{str}$ as a base-10 integer $K$. Returns $S$ repeated $K$ times. If $K \le 0$ or if $K_{str}$ is not a valid integer format (an optional leading - followed by standard decimal digits), returns "".
     • slice(Expr_str, Expr_start, Expr_end): Evaluates Expr_str to string $S$ of length $N$, Expr_start to integer $A$, and Expr_end to integer $B$ (invalid integer strings default to $0$). Computes 0-based indexing:
       • If $A < 0$, $A = \max(0, N + A)$; else $A = \min(N, A)$.
       • If $B < 0$, $B = \max(0, N + B)$; else $B = \min(N, B)$.
       • If $A \ge B$, returns "".
       • Otherwise, returns the substring from index $A$ up to index $B - 1$.
     • reverse(Expr_str): Evaluates Expr_str and returns the reversed string.

Given a string expr representing a single expression, parse and evaluate it to return the output string.