2 files changed, 73 insertions, 0 deletions
diff --git a/src/util/classify_character.rs b/src/util/classify_character.rs
new file mode 100644
index 0000000..b938502
--- /dev/null
+++ b/src/util/classify_character.rs
@@ -0,0 +1,72 @@
+//! Utilities to classify characters as whitespace, punctuation, or rest.
+
+use crate::unicode::PUNCTUATION;
+
+/// Character kinds.
+#[derive(Debug, PartialEq, Eq)]
+pub enum Kind {
+    /// Whitespace.
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | **a_b_ c**.
+    ///    ^      ^    ^
+    /// ```
+    Whitespace,
+    /// Punctuation.
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | **a_b_ c**.
+    ///     ^^ ^ ^    ^
+    /// ```
+    Punctuation,
+    /// Everything else.
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | **a_b_ c**.
+    ///       ^ ^  ^
+    /// ```
+    Other,
+}
+
+/// Classify whether a character code represents whitespace, punctuation, or
+/// something else.
+///
+/// Used for attention (emphasis, strong), whose sequences can open or close
+/// based on the class of surrounding characters.
+///
+/// > 👉 **Note** that eof (`None`) is seen as whitespace.
+///
+/// ## References
+///
+/// *   [`micromark-util-classify-character` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-util-classify-character/dev/index.js)
+pub fn classify(char: char) -> Kind {
+    // Unicode whitespace.
+    if char.is_whitespace() {
+        Kind::Whitespace
+    }
+    // Unicode punctuation.
+    else if PUNCTUATION.contains(&char) {
+        Kind::Punctuation
+    }
+    // Everything else.
+    else {
+        Kind::Other
+    }
+}
+
+/// Like [`classify`], but supports eof as whitespace.
+pub fn classify_opt(char_opt: Option<char>) -> Kind {
+    if let Some(char) = char_opt {
+        classify(char)
+    }
+    // EOF.
+    else {
+        Kind::Whitespace
+    }
+}
diff --git a/src/util/mod.rs b/src/util/mod.rs
index f51845c..022c7d6 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -1,5 +1,6 @@
 //! Utilities used when processing markdown.
 
+pub mod classify_character;
 pub mod decode_character_reference;
 pub mod edit_map;
 pub mod encode;