diff --git a/src/function.rs b/src/function.rs
new file mode 100644
index 0000000..7b0105d
--- /dev/null
+++ b/src/function.rs
@@ -0,0 +1,115 @@
+
+use std::path::{Path, PathBuf};
+use std::str;
+
+use regex::Regex;
+use nom::{IError, IResult};
+
+use super::Error;
+
+#[derive(Debug)]
+pub enum ComparisonOperator {
+    Equal,
+    NotEqual,
+    LessThan,
+    GreaterThan,
+    LessThanOrEqual,
+    GreaterThanOrEqual,
+}
+
+#[derive(Debug)]
+pub enum Function {
+    FilePath(PathBuf),
+    FileRegex(Regex),
+    ActivePath(PathBuf),
+    ActiveRegex(Regex),
+    Many(Regex),
+    ManyActive(Regex),
+    Checksum(PathBuf, u32),
+    Version(PathBuf, String, ComparisonOperator),
+}
+
+const INVALID_PATH_CHARS: &str = "\":*?<>|\\"; // \ is treated as invalid to distinguish regex strings.
+const INVALID_REGEX_PATH_CHARS: &str = "\"<>";
+
+impl Function {
+    pub fn eval(&self) -> Result<bool, Error> {
+        // TODO: Handle all variants.
+        // TODO: Paths may not lead outside game directory.
+        match *self {
+            Function::FilePath(ref f) => Ok(f.exists()),
+            _ => Ok(false),
+        }
+    }
+
+    pub fn parse(input: &str) -> IResult<&str, Function> {
+        // TODO: Handle all variants.
+        // TODO: Paths may not contain :*?"<>|
+        do_parse!(
+            input,
+            function: alt!(
+                delimited!(tag!("file(\""), is_not!(INVALID_PATH_CHARS), tag!("\")")) => {
+                    |path| Function::FilePath(PathBuf::from(path))
+                } |
+                delimited!(tag!("active(\""), is_not!(INVALID_PATH_CHARS), tag!("\")")) => {
+                    |path| Function::ActivePath(PathBuf::from(path))
+                }
+            ) >> (function)
+        )
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn function_parse_should_parse_a_file_path_function() {
+        let result = Function::parse("file(\"Cargo.toml\")").to_result().unwrap();
+
+        match result {
+            Function::FilePath(f) => assert_eq!(PathBuf::from("Cargo.toml"), f),
+            _ => panic!("Expected a file path function"),
+        }
+    }
+
+    #[test]
+    fn function_parse_should_parse_an_active_function() {
+        let result = Function::parse("active(\"Cargo.toml\")").to_result().unwrap();
+
+        match result {
+            Function::ActivePath(f) => assert_eq!(PathBuf::from("Cargo.toml"), f),
+            _ => panic!("Expected an active path function"),
+        }
+    }
+
+    #[test]
+    fn function_file_path_eval_should_return_true_if_the_file_exists_relative_to_the_data_path() {
+        let function = Function::FilePath(PathBuf::from("Cargo.toml"));
+
+        assert!(function.eval().unwrap());
+
+        unimplemented!("not yet any way to actually specify the data path");
+    }
+
+    #[test]
+    fn function_file_path_eval_should_return_true_if_given_a_plugin_that_is_ghosted() {
+        let function = Function::FilePath(PathBuf::from("test.esp"));
+
+        assert!(function.eval().unwrap());
+
+        unimplemented!("need to add tempdir and create a test.esp.ghost");
+    }
+
+    #[test]
+    fn function_file_path_eval_should_error_if_the_path_is_outside_game_directory() {
+        unimplemented!("to do");
+    }
+
+    #[test]
+    fn function_file_path_eval_should_return_false_if_the_file_does_not_exist() {
+        let function = Function::FilePath(PathBuf::from("missing"));
+
+        assert!(!function.eval().unwrap());
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index d0eb25a..d604dd6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,14 +2,15 @@
 extern crate nom;
 extern crate regex;
 
-use regex::Regex;
-
 use nom::{IError, IResult};
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 use std::str;
 
+mod function;
+use function::Function;
+
 #[derive(Debug)]
-enum Error {
+pub enum Error {
     ParsingIncomplete,
     ParsingError,
     InvalidPath(PathBuf),
@@ -25,57 +26,12 @@ impl From<IError> for Error {
     }
 }
 
-#[derive(Debug)]
-enum ComparisonOperator {
-    Equal,
-    NotEqual,
-    LessThan,
-    GreaterThan,
-    LessThanOrEqual,
-    GreaterThanOrEqual,
-}
-
-#[derive(Debug)]
-enum Function {
-    FilePath(PathBuf),
-    FileRegex(Regex),
-    ActivePath(PathBuf),
-    ActiveRegex(Regex),
-    Many(Regex),
-    ManyActive(Regex),
-    Checksum(PathBuf, u32),
-    Version(PathBuf, String, ComparisonOperator),
-}
-
-impl Function {
-    fn eval(&self) -> Result<bool, Error> {
-        // TODO: Handle all variants.
-        // TODO: Paths may not lead outside game directory.
-        match *self {
-            Function::FilePath(ref f) => Ok(f.exists()),
-            _ => Ok(false),
-        }
-    }
-
-    fn parse(input: &str) -> IResult<&str, Function> {
-        // TODO: Handle all variants.
-        // TODO: Paths may not contain :*?"<>|
-        do_parse!(
-            input,
-            tag!("file(\"")
-                >> path: is_not!("\"")
-                >> tag!("\")")
-                >> (Function::FilePath(PathBuf::from(path)))
-        )
-    }
-}
-
 // Compound conditions joined by 'or'
 #[derive(Debug)]
-struct Expression(Vec<CompoundCondition>);
+pub struct Expression(Vec<CompoundCondition>);
 
 impl Expression {
-    fn eval(&self) -> Result<bool, Error> {
+    pub fn eval(&self) -> Result<bool, Error> {
         for compound_condition in &self.0 {
             if compound_condition.eval()? {
                 return Ok(true);
@@ -84,7 +40,7 @@ impl Expression {
         Ok(false)
     }
 
-    fn parse(input: &str) -> IResult<&str, Expression> {
+    pub fn parse(input: &str) -> IResult<&str, Expression> {
         do_parse!(
             input,
             compound_conditions:
@@ -268,46 +224,6 @@ mod tests {
         }
     }
 
-    #[test]
-    fn function_parse_should_parse_a_file_function() {
-        let result = Function::parse("file(\"Cargo.toml\")").to_result().unwrap();
-
-        match result {
-            Function::FilePath(f) => assert_eq!(PathBuf::from("Cargo.toml"), f),
-            _ => panic!("Expected a file function"),
-        }
-    }
-
-    #[test]
-    fn function_file_path_eval_should_return_true_if_the_file_exists_relative_to_the_data_path() {
-        let function = Function::FilePath(PathBuf::from("Cargo.toml"));
-
-        assert!(function.eval().unwrap());
-
-        unimplemented!("not yet any way to actually specify the data path");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_return_true_if_given_a_plugin_that_is_ghosted() {
-        let function = Function::FilePath(PathBuf::from("test.esp"));
-
-        assert!(function.eval().unwrap());
-
-        unimplemented!("need to add tempdir and create a test.esp.ghost");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_error_if_the_path_is_outside_game_directory() {
-        unimplemented!("to do");
-    }
-
-    #[test]
-    fn function_file_path_eval_should_return_false_if_the_file_does_not_exist() {
-        let function = Function::FilePath(PathBuf::from("missing"));
-
-        assert!(!function.eval().unwrap());
-    }
-
     #[test]
     fn condition_eval_should_return_function_eval_for_a_function_condition() {
         let condition = Condition::Function(Function::FilePath(PathBuf::from("Cargo.toml")));