diff --git a/README.md b/README.md
index 1ee35b1..f0e2e45 100644
--- a/README.md
+++ b/README.md
@@ -29,7 +29,7 @@ This GitHub Action will expose the slug/short values of [some GitHub environment
 `SLUG` on a variable will
 
 - put the variable content in lower case
-- replace any character by `-` except `0-9`, `a-z`, and `.`
+- replace any character by `-` except `0-9`, `a-z`, `.`, and `_`
 - remove leading and trailing `-` character
 - limit the string size to 63 characters
 
diff --git a/__tests__/slugref.test.ts b/__tests__/slugref.test.ts
index 728937e..f8b08bf 100644
--- a/__tests__/slugref.test.ts
+++ b/__tests__/slugref.test.ts
@@ -16,13 +16,13 @@ test('slug_ref:: master branch', () => {
 })
 
 test('slug_ref: a feature branch', () => {
-  test_slugref('refs/heads/feat/new_feature', 'feat-new-feature')
-  test_slugref_cs('refs/heads/feat/new_feature', 'feat-new-feature')
+  test_slugref('refs/heads/feat/new_feature', 'feat-new_feature')
+  test_slugref_cs('refs/heads/feat/new_feature', 'feat-new_feature')
 })
 
 test('slug_ref: a fix branch', () => {
-  test_slugref('refs/heads/fix/issue_number', 'fix-issue-number')
-  test_slugref_cs('refs/heads/fix/issue_number', 'fix-issue-number')
+  test_slugref('refs/heads/fix/issue_number', 'fix-issue_number')
+  test_slugref_cs('refs/heads/fix/issue_number', 'fix-issue_number')
 })
 
 test('slug_ref: a simple tag', () => {
@@ -36,8 +36,8 @@ test('slug_ref: a complex tag', () => {
 })
 
 test('slug_ref: a reference with upper case letters', () => {
-  test_slugref('refs/heads/New_Awesome_Product', 'new-awesome-product')
-  test_slugref_cs('refs/heads/New_Awesome_Product', 'New-Awesome-Product')
+  test_slugref('refs/heads/New_Awesome_Product', 'new_awesome_product')
+  test_slugref_cs('refs/heads/New_Awesome_Product', 'New_Awesome_Product')
 })
 
 test('slug_ref: test trailing', () => {
diff --git a/src/slug.ts b/src/slug.ts
index fee84c8..7919bc9 100644
--- a/src/slug.ts
+++ b/src/slug.ts
@@ -2,7 +2,7 @@ const MAX_SLUG_STRING_SIZE = 63
 
 /**
  * slug_cs will take envVar and then :
- * - replace any character by `-` except `0-9`, `a-z`, and `.`
+ * - replace any character by `-` except `0-9`, `a-z`, `.`, and `_`
  * - remove leading and trailing `-` character
  * - limit the string size to 63 characters
  * @param envVar to be slugged
@@ -17,7 +17,7 @@ export function slug_cs(envVar: string): string {
 /**
  * slug will take envVar and then :
  * - put the variable content in lower case
- * - replace any character by `-` except `0-9`, `a-z`, and `.`
+ * - replace any character by `-` except `0-9`, `a-z`, `.`, and `_`
  * - remove leading and trailing `-` character
  * - limit the string size to 63 characters
  * @param envVar to be slugged
@@ -29,7 +29,7 @@ export function slug(envVar: string): string {
 /**
  * slugref_cs will take envVar and then :
  * - remove refs/(heads|tags|pull)/
- * - replace any character by `-` except `0-9`, `a-z`, and `.`
+ * - replace any character by `-` except `0-9`, `a-z`, `.`, and `_`
  * - remove leading and trailing `-` character
  * - limit the string size to 63 characters
  * @param envVar to be slugged
@@ -42,7 +42,7 @@ export function slugref_cs(envVar: string): string {
  * slugref will take envVar and then :
  * - remove refs/(heads|tags|pull)/
  * - put the variable content in lower case
- * - replace any character by `-` except `0-9`, `a-z`, and `.`
+ * - replace any character by `-` except `0-9`, `a-z`, `.`, and `_`
  * - remove leading and trailing `-` character
  * - limit the string size to 63 characters
  * @param envVar to be slugged
@@ -59,7 +59,7 @@ export function slugref(envVar: string): string {
  * @param envVar to be slugged
  */
 export function slugurl_cs(envVar: string): string {
-  return slug_cs(replaceAnyDotToHyphen(envVar))
+  return slug_cs(replaceAnyNonUrlCharactersWithHyphen(envVar))
 }
 
 /**
@@ -71,7 +71,7 @@ export function slugurl_cs(envVar: string): string {
  * @param envVar to be slugged
  */
 export function slugurl(envVar: string): string {
-  return slug(replaceAnyDotToHyphen(envVar))
+  return slug(replaceAnyNonUrlCharactersWithHyphen(envVar))
 }
 
 /**
@@ -104,11 +104,11 @@ function trailHyphen(envVar: string): string {
 }
 
 function replaceAnyNonAlphanumericCharacter(envVar: string): string {
-  return envVar.replace(RegExp('[^a-zA-Z0-9.]', 'g'), '-')
+  return envVar.replace(RegExp('[^a-zA-Z0-9._]', 'g'), '-')
 }
 
-function replaceAnyDotToHyphen(envVar: string): string {
-  return envVar.replace(RegExp('[.]', 'g'), '-')
+function replaceAnyNonUrlCharactersWithHyphen(envVar: string): string {
+  return envVar.replace(RegExp('[._]', 'g'), '-')
 }
 
 function removeRef(envVar: string): string {