diff --git a/io/util.go b/io/util.go
new file mode 100644
index 00000000..3ca4515c
--- /dev/null
+++ b/io/util.go
@@ -0,0 +1,149 @@
+package io
+
+import "io"
+
+// trieNode is a simple implementation of a trie with values
+// of a single type. Only supports []byte keys for simplicity.
+type trieNode[V any] struct {
+	hasValue bool
+	value    V
+	children map[byte]*trieNode[V]
+}
+
+func (tn *trieNode[V]) Set(key []byte, val V) {
+	// Written iteratively rather than recursively for performance
+	// reasons and to avoid OOM errors.
+	currTn := tn
+	var nextTn *trieNode[V]
+	exists := false
+
+	// Iterate down to the requested node, creating nodes
+	// along the way if necessary.
+	for i := 0; i < len(key); i++ {
+		nextTn, exists = currTn.children[key[i]]
+		if !exists {
+			nextTn = newTrieNode[V]()
+			currTn.children[key[i]] = nextTn
+		}
+		currTn = nextTn
+	}
+
+	currTn.hasValue = true
+	currTn.value = val
+}
+
+func newTrieNode[V any]() *trieNode[V] {
+	return &trieNode[V]{
+		hasValue: false,
+		children: make(map[byte]*trieNode[V]),
+	}
+}
+
+// A ReplacingReader wraps an io.Reader, performing the requested
+// replacements to its input.
+//
+// Matches the strings to be replaced in a greedy manner.
+type ReplacingReader struct {
+	innerR  io.Reader
+	buf     []byte
+	atEOF   bool
+	replace *trieNode[[]byte]
+}
+
+// Wrap r, replacing all occurences of the keys in replace
+// with their associated values.
+func NewReplacingReader(r io.Reader, replace map[string]string) io.Reader {
+	res := &ReplacingReader{
+		innerR: r,
+	}
+
+	// Build the replacement trie.
+	res.replace = newTrieNode[[]byte]()
+	for key, val := range replace {
+		res.replace.Set([]byte(key), []byte(val))
+	}
+
+	return res
+}
+
+// Wrap r, normalizing all newlines ("\n", "\r\n", or "\r") to '\n'.
+func NewNewlineNormalizingReader(r io.Reader) io.Reader {
+	return NewReplacingReader(r, map[string]string{
+		"\n":   "\n",
+		"\r":   "\n",
+		"\r\n": "\n",
+	})
+}
+
+// Reads data into p, (greedily) performing the requested replacements.
+func (rr *ReplacingReader) Read(p []byte) (n int, err error) {
+	// If we have any data in the buffer left over from the last call,
+	// read it into p and return.
+	if len(rr.buf) != 0 {
+		n = copy(p, rr.buf)
+
+		// Clear the buffer if we were able to read everything.
+		// Otherwise, advance the buffer.
+		if n < len(rr.buf) {
+			rr.buf = rr.buf[n:]
+		} else {
+			rr.buf = nil
+		}
+
+		return
+	}
+
+	// If we've encountered an io.EOF, return an io.EOF.
+	if rr.atEOF {
+		return 0, io.EOF
+	}
+
+	// Read from the underlying reader.
+	innerP := make([]byte, len(p))
+	innerN, innerErr := rr.innerR.Read(innerP)
+	if innerErr == io.EOF {
+		rr.atEOF = true
+	} else if innerErr != nil {
+		return 0, innerErr
+	}
+
+	// Copy data into p making the necessary replacements.
+	n = 0
+	i := 0
+	for n < len(p) && i < innerN {
+		// Match the longest possible replacement.
+		currTn := rr.replace
+		exists := true
+		var lastReplacement []byte
+		matchSize := 0
+		for j := 0; exists; j++ {
+			if currTn.hasValue {
+				lastReplacement = currTn.value
+				matchSize = j
+			}
+			currTn, exists = currTn.children[innerP[i+j]]
+		}
+
+		// If we found a match, copy the replacement data into p
+		// and skip what was matched. Otherwise, copy a single byte.
+		if matchSize > 0 {
+			for j, b := range lastReplacement {
+				// If we run out of space in p to put the replacement into,
+				// store the remainder in the buffer and return.
+				if n == len(p) {
+					rr.buf = lastReplacement[j:]
+					return
+				}
+
+				p[n] = b
+				n++
+			}
+			i = i + matchSize
+		} else {
+			p[n] = innerP[i]
+			n++
+			i++
+		}
+	}
+	return
+}
diff --git a/io/util_test.go b/io/util_test.go
new file mode 100644
index 00000000..2711f557
--- /dev/null
+++ b/io/util_test.go
@@ -0,0 +1,103 @@
+package io_test
+
+import (
+	"bytes"
+	"io"
+	"testing"
+
+	pio "github.com/TimothyStiles/poly/io"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestReplacingReader(t *testing.T) {
+	testCases := []struct {
+		name    string
+		input   string
+		replace map[string]string
+		want    string
+	}{{
+		name:    "does nothing to an empty string",
+		input:   "",
+		replace: map[string]string{"hi": "hey"},
+		want:    "",
+	}, {
+		name:    "performs a single replacement",
+		input:   "replace me please",
+		replace: map[string]string{"replace me": "replaced"},
+		want:    "replaced please",
+	}, {
+		name:    "does nothing when no replacement provided",
+		input:   "do not replace me please",
+		replace: map[string]string{},
+		want:    "do not replace me please",
+	}, {
+		name:    "makes multiple replacements",
+		input:   "replace me please, replace me again",
+		replace: map[string]string{"replace me": "replaced"},
+		want:    "replaced please, replaced again",
+	}, {
+		name:    "matches greedily",
+		input:   "replace me, no actually replace me instead",
+		replace: map[string]string{"replace me": "short", "replace me instead": "long"},
+		want:    "short, no actually long",
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			r := bytes.NewReader([]byte(tc.input))
+			rr := pio.NewReplacingReader(r, tc.replace)
+
+			res, err := io.ReadAll(rr)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			got := string(res)
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Errorf("unexpected output (-want,+got): %v", diff)
+			}
+		})
+	}
+}
+
+func TestNewlineNormalizingReader(t *testing.T) {
+	testCases := []struct {
+		name  string
+		input string
+		want  string
+	}{{
+		name:  "does nothing to an empty string",
+		input: "",
+		want:  "",
+	}, {
+		name:  "conserves \n newlines",
+		input: "hi\ntest\nagain\n",
+		want:  "hi\ntest\nagain\n",
+	}, {
+		name:  "normalizes \r newlines",
+		input: "hi\ntest\ragain\n",
+		want:  "hi\ntest\nagain\n",
+	}, {
+		name:  "normalizes \r\n newlines",
+		input: "hi\ntest\r\nagain\n",
+		want:  "hi\ntest\nagain\n",
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			r := bytes.NewReader([]byte(tc.input))
+			rr := pio.NewNewlineNormalizingReader(r)
+
+			res, err := io.ReadAll(rr)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			got := string(res)
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Errorf("unexpected output (-want,+got): %v", diff)
+			}
+		})
+	}
+
+}