[CMake] What does this mean: IF("${VARIABLE}" MATCHES "^${VARIABLE}$")

[Clark J. Wang]

The header of many CMake modules often looks like this (take
CheckIncludeFile as an example):

MACRO(CHECK_INCLUDE_FILE INCLUDE VARIABLE)
  IF("${VARIABLE}" MATCHES "^${VARIABLE}$")
  ... ...

The IF statement really puzzles me a lot. What is it used to do?
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