diff --git a/src/lib.rs b/src/lib.rs
index c4fcfd6..ce95968 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -18,18 +18,17 @@
 //! ```
 
 mod render;
+mod slug;
 
 use std::borrow::Borrow;
-use std::collections::HashMap;
 use std::fmt::Write;
 use std::slice::Iter;
 
-use once_cell::sync::Lazy;
 pub use pulldown_cmark::HeadingLevel;
 use pulldown_cmark::{Event, Options as CmarkOptions, Parser, Tag};
-use regex::Regex;
 
 pub use render::{ItemSymbol, Options};
+pub use slug::{GitHubSlugifier, Slugify};
 
 /////////////////////////////////////////////////////////////////////////
 // Definitions
@@ -77,15 +76,6 @@ impl Heading<'_> {
         }
         buf
     }
-
-    /// Generate an anchor link for this heading.
-    ///
-    /// This is calculated in the same way that GitHub calculates it.
-    pub fn anchor(&self) -> String {
-        static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"[^\w\- ]").unwrap());
-        RE.replace_all(&self.text().to_ascii_lowercase().replace(' ', "-"), "")
-            .into_owned()
-    }
 }
 
 impl<'a> TableOfContents<'a> {
@@ -212,26 +202,15 @@ impl<'a> TableOfContents<'a> {
             item_symbol,
             levels,
             indent,
+            slugifier: mut slugger,
         } = options;
 
-        // this is to record duplicates
-        let mut counts = HashMap::new();
-
         let mut buf = String::new();
         for heading in self.headings().filter(|h| levels.contains(&h.level())) {
             let title = crate::render::to_cmark(heading.events());
-            let anchor = heading.anchor();
             let indent = indent * (heading.level() as usize - *levels.start() as usize);
 
             // make sure the anchor is unique
-            let i = counts
-                .entry(anchor.clone())
-                .and_modify(|i| *i += 1)
-                .or_insert(0);
-            let anchor = match *i {
-                0 => anchor,
-                i => format!("{}-{}", anchor, i),
-            };
 
             writeln!(
                 buf,
@@ -239,7 +218,7 @@ impl<'a> TableOfContents<'a> {
                 "",
                 item_symbol,
                 title,
-                anchor,
+                slugger.slugify(&heading.text()),
                 indent = indent,
             )
             .unwrap();
@@ -278,25 +257,6 @@ mod tests {
         assert_eq!(heading.text(), "Here TOML");
     }
 
-    #[test]
-    fn heading_anchor_with_code() {
-        let heading = Heading {
-            events: vec![Code(Borrowed("Another")), Text(Borrowed(" heading"))],
-            level: HeadingLevel::H1,
-        };
-        assert_eq!(heading.anchor(), "another-heading");
-    }
-
-    #[test]
-    fn heading_anchor_with_links() {
-        let events = Parser::new("Here [TOML](https://toml.io)").collect();
-        let heading = Heading {
-            events,
-            level: HeadingLevel::H1,
-        };
-        assert_eq!(heading.anchor(), "here-toml");
-    }
-
     #[test]
     fn toc_new() {
         let toc = TableOfContents::new("# Heading\n\n## `Another` heading\n");
diff --git a/src/render.rs b/src/render.rs
index d1b0346..4a0d689 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -7,6 +7,8 @@ use std::ops::RangeInclusive;
 
 use pulldown_cmark::{Event, HeadingLevel, Tag};
 
+use crate::slug::{GitHubSlugifier, Slugify};
+
 /// Which symbol to use when rendering Markdown list items.
 pub enum ItemSymbol {
     /// `-`
@@ -32,6 +34,7 @@ pub struct Options {
     pub(crate) item_symbol: ItemSymbol,
     pub(crate) levels: RangeInclusive<HeadingLevel>,
     pub(crate) indent: usize,
+    pub(crate) slugifier: Box<dyn Slugify>,
 }
 
 pub(crate) fn to_cmark<'a, I, E>(events: I) -> String
@@ -73,6 +76,7 @@ impl Default for Options {
             item_symbol: ItemSymbol::Hyphen,
             levels: (HeadingLevel::H1..=HeadingLevel::H6),
             indent: 2,
+            slugifier: Box::new(GitHubSlugifier::default()),
         }
     }
 }
@@ -98,4 +102,11 @@ impl Options {
         self.indent = indent;
         self
     }
+
+    /// The slugifier to use for the heading anchors.
+    #[must_use]
+    pub fn slugifier(mut self, slugifier: Box<dyn Slugify>) -> Self {
+        self.slugifier = slugifier;
+        self
+    }
 }
diff --git a/src/slug.rs b/src/slug.rs
new file mode 100644
index 0000000..d5acbb2
--- /dev/null
+++ b/src/slug.rs
@@ -0,0 +1,77 @@
+use std::{borrow::Cow, collections::HashMap};
+
+use once_cell::sync::Lazy;
+use regex::Regex;
+
+/// A trait to specify the anchor calculation.
+pub trait Slugify {
+    fn slugify<'a>(&mut self, str: &'a str) -> Cow<'a, str>;
+}
+
+/// A slugifier that attempts to mimic GitHub's behavior.
+///
+/// Unfortunately GitHub's behavior is not documented anywhere by GitHub.
+/// This should really be part of the [GitHub Flavored Markdown Spec][gfm]
+/// but alas it's not. And there also does not appear to be a public issue
+/// tracker for the spec where that issue could be raised.
+///
+/// [gfm]: https://github.github.com/gfm/
+#[derive(Default)]
+pub struct GitHubSlugifier {
+    counts: HashMap<String, i32>,
+}
+
+impl Slugify for GitHubSlugifier {
+    fn slugify<'a>(&mut self, str: &'a str) -> Cow<'a, str> {
+        static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"[^\w\- ]").unwrap());
+        let anchor = RE
+            .replace_all(&str.to_ascii_lowercase().replace(' ', "-"), "")
+            .into_owned();
+
+        let i = self
+            .counts
+            .entry(anchor.clone())
+            .and_modify(|i| *i += 1)
+            .or_insert(0);
+
+        match *i {
+            0 => anchor,
+            i => format!("{}-{}", anchor, i),
+        }
+        .into()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::slug::{GitHubSlugifier, Slugify};
+    use crate::Heading;
+    use pulldown_cmark::CowStr::Borrowed;
+    use pulldown_cmark::Event::{Code, Text};
+    use pulldown_cmark::{HeadingLevel, Parser};
+
+    #[test]
+    fn heading_anchor_with_code() {
+        let heading = Heading {
+            events: vec![Code(Borrowed("Another")), Text(Borrowed(" heading"))],
+            level: HeadingLevel::H1,
+        };
+        assert_eq!(
+            GitHubSlugifier::default().slugify(&heading.text()),
+            "another-heading"
+        );
+    }
+
+    #[test]
+    fn heading_anchor_with_links() {
+        let events = Parser::new("Here [TOML](https://toml.io)").collect();
+        let heading = Heading {
+            events,
+            level: HeadingLevel::H1,
+        };
+        assert_eq!(
+            GitHubSlugifier::default().slugify(&heading.text()),
+            "here-toml"
+        );
+    }
+}