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# CMP0053

Simplify variable reference and escape sequence evaluation.

CMake 3.1 introduced a much faster implementation of evaluation of the Variable References and Escape Sequences documented in the `cmake-language(7)` manual. While the behavior is identical to the legacy implementation in most cases, some corner cases were cleaned up to simplify the behavior. Specifically:

- Expansion of `@VAR@` reference syntax defined by the `configure_file()` and `string(CONFIGURE)` commands is no longer performed in other contexts.

- Literal `${VAR}` reference syntax may contain only alphanumeric characters (`A-Z`, `a-z`, `0-9`) and the characters `_`, `.`, `/`, `-`, and `+`. Variables with other characters in their name may still be referenced indirectly, e.g.

      set(varname "otherwise & disallowed $ characters")
      message("${${varname}}")

- The setting of policy `CMP0010` is not considered, so improper variable reference syntax is always an error.

- More characters are allowed to be escaped in variable names. Previously, only `()#" \@^` were valid characters to escape. Now any non-alphanumeric, non-semicolon, non-NUL character may be escaped following the `escape_identity` production in the Escape Sequences section of the `cmake-language(7)` manual.

The `OLD` behavior for this policy is to honor the legacy behavior for variable references and escape sequences. The `NEW` behavior is to use the simpler variable expansion and escape sequence evaluation rules.

This policy was introduced in CMake version 3.1. CMake version 3.1.2 warns when the policy is not set and uses `OLD` behavior. Use the `cmake_policy()` command to set it to `OLD` or `NEW` explicitly.

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