diff --git a/bin/git-delete-merged-branches b/bin/git-delete-merged-branches
index 38988cf..1721f9d 100755
--- a/bin/git-delete-merged-branches
+++ b/bin/git-delete-merged-branches
@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 
-branches=$(git branch --no-color --merged | grep -vE "^(\*|\+)" | grep -v "$(git_extra_default_branch)" | grep -v svn)
+default_branch=$(git_extra_default_branch)
+branches=$(git branch --no-color --merged | sed 's/^[ *+]*//' | grep -Fvx -e "$default_branch" -e "main" -e "master" -e "trunk" -e "svn")
 if [ -n "$branches" ]
 then
     echo "$branches" | xargs git branch -d
diff --git a/bin/git-show-merged-branches b/bin/git-show-merged-branches
index 6b18c78..d959f4e 100755
--- a/bin/git-show-merged-branches
+++ b/bin/git-show-merged-branches
@@ -1,3 +1,4 @@
 #!/usr/bin/env bash
 
-git branch --no-color --merged | grep -v "\*" | grep -v "$(git_extra_default_branch)" | tr -d ' '
+default_branch=$(git_extra_default_branch)
+git branch --no-color --merged | sed 's/^[ *+]*//' | grep -Fvx -e "$default_branch" -e "main" -e "master" -e "trunk" -e "svn"
diff --git a/bin/git-show-unmerged-branches b/bin/git-show-unmerged-branches
index 5c4e1ec..5859aff 100755
--- a/bin/git-show-unmerged-branches
+++ b/bin/git-show-unmerged-branches
@@ -1,3 +1,4 @@
 #!/usr/bin/env bash
 
-git branch --no-color --no-merged | grep -v "\*" | grep -v "$(git_extra_default_branch)" | tr -d ' '
+default_branch=$(git_extra_default_branch)
+git branch --no-color --no-merged | sed 's/^[ *+]*//' | grep -Fvx -e "$default_branch" -e "main" -e "master" -e "trunk" -e "svn"
diff --git a/helper/git-extra-utility b/helper/git-extra-utility
index 166d074..6ed6943 100755
--- a/helper/git-extra-utility
+++ b/helper/git-extra-utility
@@ -13,6 +13,14 @@ git_extra_default_branch() {
         echo "$extras_default_branch"
     elif [ -n "$init_default_branch" ]; then
         echo "$init_default_branch"
+    elif git show-ref --verify --quiet refs/heads/main; then
+        echo "main"
+    elif git show-ref --verify --quiet refs/heads/master; then
+        echo "master"
+    elif git show-ref --verify --quiet refs/heads/trunk; then
+        echo "trunk"
+    elif git show-ref --verify --quiet refs/heads/svn; then
+        echo "svn"
     else
         echo "main"
     fi
diff --git a/tests/git-delete-merged-branches.bats b/tests/git-delete-merged-branches.bats
new file mode 100644
index 0000000..c5b5426
--- /dev/null
+++ b/tests/git-delete-merged-branches.bats
@@ -0,0 +1,70 @@
+# shellcheck shell=bash
+
+source "$BATS_TEST_DIRNAME/test_util.sh"
+
+setup_file() {
+	test_util.setup_file
+	test_util.install_command delete-merged-branches
+	test_util.install_command show-merged-branches
+	test_util.install_command show-unmerged-branches
+}
+
+setup() {
+	test_util.cd_test
+
+	test_util.git_init
+	git commit --allow-empty -m "Initial commit"
+	git branch merged-branch
+	git branch unmerged-branch
+	git checkout unmerged-branch
+	git commit --allow-empty -m "Unmerged commit"
+	git checkout main
+}
+
+@test "show-merged-branches lists merged branches but not default branch" {
+	run git show-merged-branches
+	assert_output "merged-branch"
+	assert_success
+}
+
+@test "show-unmerged-branches lists unmerged branches" {
+	run git show-unmerged-branches
+	assert_output "unmerged-branch"
+	assert_success
+}
+
+@test "delete-merged-branches deletes merged branches and preserves default and unmerged branches" {
+	run git delete-merged-branches
+	assert_success
+
+	run git branch --list
+	assert_line -p "main"
+	assert_line -p "unmerged-branch"
+	refute_line -p "merged-branch"
+}
+
+@test "delete-merged-branches when checked out on feature branch protects default branch" {
+	git branch merged-feature
+	git checkout unmerged-branch
+	run git delete-merged-branches
+	assert_success
+
+	run git branch --list
+	assert_line -p "main"
+	assert_line -p "unmerged-branch"
+	refute_line -p "merged-feature"
+}
+
+@test "delete-merged-branches handles branch names with special characters" {
+	git branch "feature/foo+bar"
+	run git show-merged-branches
+	assert_line -p "feature/foo+bar"
+	assert_success
+
+	run git delete-merged-branches
+	assert_success
+
+	run git branch --list
+	assert_line -p "main"
+	refute_line -p "feature/foo+bar"
+}