diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index df8f540..4e0231d 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -101,6 +101,8 @@ jobs:
         run: bash actions/release/test_update_changelog.sh
       - name: Test check_test_files.py
         run: bash actions/test-sdist/test_check_test_files.sh
+      - name: Test parse_diff.sh
+        run: bash actions/pre-commit-autoupdate/test_parse_diff.sh
 
   test_pre_commit_autoupdate:
     if: github.event_name != 'schedule' || github.repository == 'Calysto/maintainer_tools'
diff --git a/actions/pre-commit-autoupdate/action.yml b/actions/pre-commit-autoupdate/action.yml
index d156f14..1b59c4b 100644
--- a/actions/pre-commit-autoupdate/action.yml
+++ b/actions/pre-commit-autoupdate/action.yml
@@ -79,19 +79,7 @@ runs:
         FULL_BRANCH="${BRANCH}-${SUFFIX}"
 
         # Parse updated packages from diff
-        UPDATES=""
-        CURRENT_REPO=""
-        OLD_REV=""
-        while IFS= read -r line; do
-          if [[ "$line" == " "* ]] && [[ "$line" =~ repo:[[:space:]]+(.*) ]]; then
-            CURRENT_REPO=$(basename "${BASH_REMATCH[1]}")
-          elif [[ "$line" =~ ^-[[:space:]]+rev:[[:space:]]+(.*) ]]; then
-            OLD_REV="${BASH_REMATCH[1]}"
-          elif [[ "$line" =~ ^\+[[:space:]]+rev:[[:space:]]+(.*) ]]; then
-            NEW_REV="${BASH_REMATCH[1]}"
-            UPDATES="${UPDATES}- ${CURRENT_REPO}: \`${OLD_REV}\` → \`${NEW_REV}\`"$'\n'
-          fi
-        done < <(git diff .pre-commit-config.yaml)
+        UPDATES=$(git diff .pre-commit-config.yaml | bash "${{ github.action_path }}/parse_diff.sh" -)
 
         BODY=""
         if [ -n "$UPDATES" ]; then
diff --git a/actions/pre-commit-autoupdate/parse_diff.sh b/actions/pre-commit-autoupdate/parse_diff.sh
new file mode 100644
index 0000000..52e445e
--- /dev/null
+++ b/actions/pre-commit-autoupdate/parse_diff.sh
@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Parse a git diff of .pre-commit-config.yaml and emit a markdown list of updates.
+# Usage: parse_diff.sh <diff-file>
+# Outputs lines of the form: "- <repo>: `<old>` → `<new>`"
+set -euo pipefail
+
+DIFF_FILE="${1:--}"
+
+CURRENT_REPO=""
+OLD_REV=""
+while IFS= read -r line; do
+  if [[ "$line" == " "* ]] && [[ "$line" =~ repo:[[:space:]]+(.*) ]]; then
+    CURRENT_REPO=$(basename "${BASH_REMATCH[1]}")
+  elif [[ "$line" =~ ^-[[:space:]]+rev:[[:space:]]+(.*) ]]; then
+    OLD_REV="${BASH_REMATCH[1]//\'/}"
+    OLD_REV="${OLD_REV//\"/}"
+  elif [[ "$line" =~ ^\+[[:space:]]+rev:[[:space:]]+(.*) ]]; then
+    NEW_REV="${BASH_REMATCH[1]//\'/}"
+    NEW_REV="${NEW_REV//\"/}"
+    echo "- ${CURRENT_REPO}: \`${OLD_REV}\` → \`${NEW_REV}\`"
+  fi
+done < <(cat "$DIFF_FILE")
diff --git a/actions/pre-commit-autoupdate/test_parse_diff.sh b/actions/pre-commit-autoupdate/test_parse_diff.sh
new file mode 100644
index 0000000..e8bbc2b
--- /dev/null
+++ b/actions/pre-commit-autoupdate/test_parse_diff.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT="$(cd "$(dirname "$0")" && pwd)/parse_diff.sh"
+FAIL=0
+
+check() {
+  local desc="$1"
+  local expected="$2"
+  local actual="$3"
+  if [ "$actual" = "$expected" ]; then
+    echo "OK: $desc"
+  else
+    echo "FAIL: $desc"
+    echo "  expected: $expected"
+    echo "  actual:   $actual"
+    FAIL=1
+  fi
+}
+
+# Simulate a git diff with three repos using no quotes, single quotes, and double quotes
+DIFF=$(cat <<'EOF'
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+-   rev: v4.4.0
++   rev: v4.5.0
+  - repo: https://github.com/psf/black
+-   rev: '23.1.0'
++   rev: '23.3.0'
+  - repo: https://github.com/PyCQA/flake8
+-   rev: "6.0.0"
++   rev: "6.1.0"
+EOF
+)
+
+ACTUAL=$(echo "$DIFF" | bash "$SCRIPT" -)
+
+check "no-quotes version" \
+  "- pre-commit-hooks: \`v4.4.0\` → \`v4.5.0\`" \
+  "$(echo "$ACTUAL" | grep pre-commit-hooks)"
+
+check "single-quoted version" \
+  "- black: \`23.1.0\` → \`23.3.0\`" \
+  "$(echo "$ACTUAL" | grep black)"
+
+check "double-quoted version" \
+  "- flake8: \`6.0.0\` → \`6.1.0\`" \
+  "$(echo "$ACTUAL" | grep flake8)"
+
+exit $FAIL