Fix a bug when there are regex characters in the path

Closes #61

cmake/cmkr.cmake

@@ -58,7 +58,7 @@ else()
 endif()
 
 # Use cached cmkr if found
-if(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}")
+if(DEFINED ENV{CMKR_CACHE})
     set(CMKR_DIRECTORY_PREFIX "$ENV{CMKR_CACHE}")
     string(REPLACE "\\" "/" CMKR_DIRECTORY_PREFIX "${CMKR_DIRECTORY_PREFIX}")
     if(NOT CMKR_DIRECTORY_PREFIX MATCHES "\\/$")
@@ -72,10 +72,27 @@ endif()
 set(CMKR_DIRECTORY "${CMKR_DIRECTORY_PREFIX}${CMKR_TAG}")
 set(CMKR_CACHED_EXECUTABLE "${CMKR_DIRECTORY}/bin/${CMKR_EXECUTABLE_NAME}")
 
+# Helper function to check if a string starts with a prefix
+# Cannot use MATCHES, see: https://github.com/build-cpp/cmkr/issues/61
+function(cmkr_startswith str prefix result)
+    string(LENGTH "${prefix}" prefix_length)
+    string(LENGTH "${str}" str_length)
+    if(prefix_length LESS_EQUAL str_length)
+        string(SUBSTRING "${str}" 0 ${prefix_length} str_prefix)
+        if(prefix STREQUAL str_prefix)
+            set("${result}" ON PARENT_SCOPE)
+            return()
+        endif()
+    endif()
+    set("${result}" OFF PARENT_SCOPE)
+endfunction()
+
 # Handle upgrading logic
 if(CMKR_EXECUTABLE AND NOT CMKR_CACHED_EXECUTABLE STREQUAL CMKR_EXECUTABLE)
-    if(CMKR_EXECUTABLE MATCHES "^${CMAKE_CURRENT_BINARY_DIR}/_cmkr")
-        if(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}")
+    cmkr_startswith("${CMKR_EXECUTABLE}" "${CMAKE_CURRENT_BINARY_DIR}/_cmkr" CMKR_STARTSWITH_BUILD)
+    cmkr_startswith("${CMKR_EXECUTABLE}" "${CMKR_DIRECTORY_PREFIX}" CMKR_STARTSWITH_CACHE)
+    if(CMKR_STARTSWITH_BUILD)
+        if(DEFINED ENV{CMKR_CACHE})
             message(AUTHOR_WARNING "[cmkr] Switching to cached cmkr: '${CMKR_CACHED_EXECUTABLE}'")
             if(EXISTS "${CMKR_CACHED_EXECUTABLE}")
                 set(CMKR_EXECUTABLE "${CMKR_CACHED_EXECUTABLE}" CACHE FILEPATH "Full path to cmkr executable" FORCE)
@@ -86,7 +103,7 @@ if(CMKR_EXECUTABLE AND NOT CMKR_CACHED_EXECUTABLE STREQUAL CMKR_EXECUTABLE)
             message(AUTHOR_WARNING "[cmkr] Upgrading '${CMKR_EXECUTABLE}' to '${CMKR_CACHED_EXECUTABLE}'")
             unset(CMKR_EXECUTABLE CACHE)
         endif()
-    elseif(DEFINED ENV{CMKR_CACHE} AND EXISTS "$ENV{CMKR_CACHE}" AND CMKR_EXECUTABLE MATCHES "^${CMKR_DIRECTORY_PREFIX}")
+    elseif(DEFINED ENV{CMKR_CACHE} AND CMKR_STARTSWITH_CACHE)
         message(AUTHOR_WARNING "[cmkr] Upgrading cached '${CMKR_EXECUTABLE}' to '${CMKR_CACHED_EXECUTABLE}'")
         unset(CMKR_EXECUTABLE CACHE)
     endif()