diff --git a/README.md b/README.md
index 69afaf4..9a9d15c 100644
--- a/README.md
+++ b/README.md
@@ -39,11 +39,14 @@ You can sponsor me [here](https://github.com/sponsors/jidicula)!
   * Available values: `LLVM`, `Google`, `Chromium`, `Mozilla`, `WebKit` and others listed in the `clang-format` [docs for BasedOnStyle](https://clang.llvm.org/docs/ClangFormatStyleOptions.html#configurable-format-style-options).
 * `exclude-regex` [optional]: A regex to exclude files or directories that should not be checked.
   * Default: `^$`
-  * Pattern matching is done with a real regex, not a glob expression. You can exclude multiple patterns like this: `(hello|world)`.
+  * Pattern matching is done with a POSIX `grep -E` extended regex, **not** a glob expression. You can exclude multiple patterns like this: `(hello|world)`. Build and verify your regex at https://regex101.com .
+* `include-regex` [optional]: A regex to include files or directories that should be checked.
+  * Default: `^.*\.((((c|C)(c|pp|xx|\+\+)?$)|((h|H)h?(pp|xx|\+\+)?$))|(ino|pde|proto|cu))$`
+  * Pattern matching is done with a POSIX `grep -E` extended regex, **not** a glob expression. You can exclude multiple patterns like this: `(hello|world)`. Build and verify your regex at https://regex101.com .
 
 This action checks all C/C++/Protobuf (including Arduino `.ino` and `.pde`) files in the provided directory in the GitHub workspace are formatted correctly using `clang-format`. If no directory is provided or the provided path is not a directory in the GitHub workspace, all C/C++/Protobuf files are checked.
 
-The following file extensions are checked:
+The following file extensions are checked by default:
 * Header files:
   * `.h`
   * `.H`
@@ -71,7 +74,7 @@ The following file extensions are checked:
 
 # Usage
 
-⚠️This action does not run on `windows`  GitHub Actions runners!
+⚠️This action does not run on `windows` GitHub Actions runners!
 
 ## Single Path
 
diff --git a/action.yml b/action.yml
index 99fd048..91817bf 100644
--- a/action.yml
+++ b/action.yml
@@ -22,12 +22,16 @@ inputs:
     description: 'The fallback style for clang-format if no .clang-format file exists in your repository.'
     required: false
     default: 'llvm'
+  include-regex:
+    description: 'A regex to override the C/C++/Protobuf/CUDA filetype regex. that should be checked. Default results in the regex defined in `check.sh`.'
+    required: false
+    default: ''
 
 runs:
   using: "composite"
   steps:
     - run: |
-        "${{ github.action_path }}/check.sh" "${{ inputs.clang-format-version }}" "${{ inputs.check-path }}" "${{ inputs.fallback-style }}" "${{ inputs.exclude-regex }}"
+        "${{ github.action_path }}/check.sh" "${{ inputs.clang-format-version }}" "${{ inputs.check-path }}" "${{ inputs.fallback-style }}" "${{ inputs.exclude-regex }} "${{ inputs.include-regex }}""
       shell: bash
     - name: Save PR head commit SHA
       if: failure() && github.event_name == 'pull_request'
diff --git a/check.sh b/check.sh
index 2bf4cdb..95cb388 100755
--- a/check.sh
+++ b/check.sh
@@ -45,12 +45,24 @@ CLANG_FORMAT_VERSION="$1"
 CHECK_PATH="$2"
 FALLBACK_STYLE="$3"
 EXCLUDE_REGEX="$4"
+INCLUDE_REGEX="$5"
 
 # Set the regex to an empty string regex if nothing was provided
-if [ -z "$EXCLUDE_REGEX" ]; then
+if [[ -z $EXCLUDE_REGEX ]]; then
 	EXCLUDE_REGEX="^$"
 fi
 
+# Set the filetype regex if nothing was provided.
+# Find all C/C++/Protobuf/CUDA files:
+#   h, H, hpp, hh, h++, hxx
+#   c, C, cpp, cc, c++, cxx
+#   ino, pde
+#   proto
+#   cu
+if [[ -z $INCLUDE_REGEX ]]; then
+	INCLUDE_REGEX='^.*\.((((c|C)(c|pp|xx|\+\+)?$)|((h|H)h?(pp|xx|\+\+)?$))|(ino|pde|proto|cu))$'
+fi
+
 cd "$GITHUB_WORKSPACE" || exit 2
 
 if [[ ! -d $CHECK_PATH ]]; then
@@ -62,13 +74,7 @@ fi
 exit_code=0
 
 # All files improperly formatted will be printed to the output.
-# find all C/C++/Protobuf/CUDA files:
-#   h, H, hpp, hh, h++, hxx
-#   c, C, cpp, cc, c++, cxx
-#   ino, pde
-#   proto
-#   cu
-src_files=$(find "$CHECK_PATH" -name .git -prune -o -regextype posix-egrep -regex '^.*\.((((c|C)(c|pp|xx|\+\+)?$)|((h|H)h?(pp|xx|\+\+)?$))|(ino|pde|proto|cu))$' -print)
+src_files=$(find "$CHECK_PATH" -name .git -prune -o -regextype posix-egrep -regex "$INCLUDE_REGEX" -print)
 
 # check formatting in each source file
 for file in $src_files; do
diff --git a/test/test.sh b/test/test.sh
index fbe1633..34dc065 100755
--- a/test/test.sh
+++ b/test/test.sh
@@ -4,10 +4,14 @@ FALLBACK_STYLE="llvm"
 EXCLUDE_REGEX="capital"
 CLANG_FORMAT_VERSION="$1"
 
+###############################################################################
+#                      Default C/C++/Protobuf/CUDA regex                      #
+###############################################################################
+
 # should succeed
 "$GITHUB_WORKSPACE"/check.sh "$CLANG_FORMAT_VERSION" "$GITHUB_WORKSPACE/test/known_pass" "$FALLBACK_STYLE" "$EXCLUDE_REGEX"
 docker_status="$?"
-if [[ "$docker_status" != "0" ]]; then
+if [[ $docker_status != "0" ]]; then
 	echo "files that should succeed have failed!"
 	exit 1
 fi
@@ -15,7 +19,29 @@ fi
 # should fail
 "$GITHUB_WORKSPACE"/check.sh "$CLANG_FORMAT_VERSION" "$GITHUB_WORKSPACE/test/known_fail" "$FALLBACK_STYLE" "$EXCLUDE_REGEX"
 docker_status="$?"
-if [[ "$docker_status" == "0" ]]; then
+if [[ $docker_status == "0" ]]; then
+	echo "files that should fail have succeeded!"
+	exit 2
+fi
+
+###############################################################################
+#                            Custom filetype regex                            #
+###############################################################################
+
+INCLUDE_REGEX='^.*\.(c|C)'
+
+# should succeed
+"$GITHUB_WORKSPACE"/check.sh "$CLANG_FORMAT_VERSION" "$GITHUB_WORKSPACE/test/known_pass" "$FALLBACK_STYLE" "$EXCLUDE_REGEX" "$INCLUDE_REGEX"
+docker_status="$?"
+if [[ $docker_status != "0" ]]; then
+	echo "files that should succeed have failed!"
+	exit 1
+fi
+
+# should fail
+"$GITHUB_WORKSPACE"/check.sh "$CLANG_FORMAT_VERSION" "$GITHUB_WORKSPACE/test/known_fail" "$FALLBACK_STYLE" "$EXCLUDE_REGEX" "$INCLUDE_REGEX"
+docker_status="$?"
+if [[ $docker_status == "0" ]]; then
 	echo "files that should fail have succeeded!"
 	exit 2
 fi