diff --git a/src/mango/io/fortest/fromstr.rs b/src/mango/io/fortest/fromstr.rs
index 3a39b80c..7c70c0d2 100644
--- a/src/mango/io/fortest/fromstr.rs
+++ b/src/mango/io/fortest/fromstr.rs
@@ -1,5 +1,5 @@
 use mango::io::typ::Reader;
-use regex::Regex;
+use mango::io::util::REXCACHE;
 
 /// Implementation of [Reader] that reads from a pre-provided string.
 /// Mostly for testing purposes.
@@ -23,7 +23,11 @@ impl Reader for StringReader {
         false
     }
 
-    fn matches(&mut self, pattern: Regex) -> Option<String> {
-        unimplemented!() // TODO
+    fn matches(&mut self, subpattern: String) -> Option<String> {
+        REXCACHE.with(|rl| {
+            let mut rexlib = rl.borrow_mut();
+            //            let rex = rexlib.make_or_get(subpattern);
+        });
+        Option::None // TODO
     }
 }
diff --git a/src/mango/io/mod.rs b/src/mango/io/mod.rs
index 6dbbd816..d9a219cf 100644
--- a/src/mango/io/mod.rs
+++ b/src/mango/io/mod.rs
@@ -1,3 +1,5 @@
 pub mod typ;
 
 pub mod fortest;
+
+pub mod util;
diff --git a/src/mango/io/typ.rs b/src/mango/io/typ.rs
index 00aa6f7a..a50008ce 100644
--- a/src/mango/io/typ.rs
+++ b/src/mango/io/typ.rs
@@ -6,7 +6,7 @@ pub trait Reader {
     fn equals(&mut self, text: &str) -> bool;
 
     /// Checks whether the code from the current position matches a regex pattern.
-    fn matches(&mut self, pattern: Regex) -> Option<String>;
+    fn matches(&mut self, subpattern: String) -> Option<String>;
 }
 
 pub trait Writer {
diff --git a/src/mango/io/util.rs b/src/mango/io/util.rs
new file mode 100644
index 00000000..e847d602
--- /dev/null
+++ b/src/mango/io/util.rs
@@ -0,0 +1,20 @@
+use regex::Regex;
+use std::cell::RefCell;
+use std::collections::HashMap;
+
+pub struct RegexCache {
+    cache: HashMap<String, Regex>,
+}
+
+impl RegexCache {
+    // Not public to prevent having more than one instance.
+    pub fn new() -> Self {
+        RegexCache {
+            cache: HashMap::new(),
+        }
+    }
+}
+
+thread_local! {
+    pub static REXCACHE: RefCell<RegexCache> = RefCell::new(RegexCache::new())
+}