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Observed behavior:

Using CMake 3.13.1, with the SAME INPUT STRING, and SAME REGEX SPECIFICATION STRING (match-regex), string(REGEX MATCH ...) is matching the expected substring, but string(REGEX REPLACE ...) is giving a warning that it only matched the empty string.

Expected behavior:

string(REGEX REPLACE ... will match the same portion of the (identical) input text, using the same regular expression, and perform the substitution.

MCVE:

Here is a minimally complete verifiable example:

cmake_minimum_required(VERSION 3.13.1)
set(MY_STRING "1;2;4;8;16
2;4;9;18;38
")
message(STATUS "MY_STRING = ${MY_STRING}")
string(REGEX MATCH "^[^\n]*" FIRST_LINE "${MY_STRING}")
message(STATUS "FIRST_LINE = ${FIRST_LINE}")
message(STATUS "MY_STRING = ${MY_STRING}")
string(REGEX REPLACE "^[^\n]*" "" REST "${MY_STRING}")
message(STATUS "REST = ${REST}")

And here is the output it yields:

-- MY_STRING = 1;2;4;8;16
2;4;9;18;38

-- FIRST_LINE = 1;2;4;8;16
-- MY_STRING = 1;2;4;8;16
2;4;9;18;38

CMake Error at CMakeLists.txt:9 (string):
  string sub-command REGEX, mode REPLACE: regex "^[^

  ]*" matched an empty string.


-- REST =
-- Configuring incomplete, errors occurred!