diff --git a/lexers/README.md b/lexers/README.md
new file mode 100644
index 0000000..43bdbbd
--- /dev/null
+++ b/lexers/README.md
@@ -0,0 +1,19 @@
+# Lexer tests
+
+The tests in this directory feed a known input `testdata/<name>.actual` into the parser for `<name>` and check
+that its output matches `<name>.exported`.
+
+## Running the tests
+
+Run the tests as normal:
+```go
+go run ./lexers
+```
+
+## Updating the existing tests
+
+You can regenerate all the test outputs
+
+```go
+RECORD=true go test ./lexers
+```
diff --git a/lexers/lexers_test.go b/lexers/lexers_test.go
index 69f6cf9..5cdbdb2 100644
--- a/lexers/lexers_test.go
+++ b/lexers/lexers_test.go
@@ -9,7 +9,6 @@ import (
 	"testing"
 
 	"github.com/alecthomas/assert"
-
 	"github.com/alecthomas/chroma"
 	"github.com/alecthomas/chroma/formatters"
 	"github.com/alecthomas/chroma/lexers"
@@ -64,15 +63,23 @@ func TestLexers(t *testing.T) {
 			actual, err := chroma.Tokenise(lexer, nil, string(actualText))
 			assert.NoError(t, err)
 
-			// Read expected JSON into token slice.
-			var expected []chroma.Token
-			r, err := os.Open(expectedFilename)
-			assert.NoError(t, err)
-			err = json.NewDecoder(r).Decode(&expected)
-			assert.NoError(t, err)
+			if os.Getenv("RECORD") == "true" {
+				// Update the expected file with the generated output of this lexer
+				f, err := os.Create(expectedFilename)
+				defer f.Close()
+				assert.NoError(t, err)
+				assert.NoError(t, formatters.JSON.Format(f, nil, chroma.Literator(actual...)))
+			} else {
+				// Read expected JSON into token slice.
+				var expected []chroma.Token
+				r, err := os.Open(expectedFilename)
+				assert.NoError(t, err)
+				err = json.NewDecoder(r).Decode(&expected)
+				assert.NoError(t, err)
 
-			// Equal?
-			assert.Equal(t, expected, actual)
+				// Equal?
+				assert.Equal(t, expected, actual)
+			}
 		})
 	}
 }
diff --git a/lexers/testdata/README.md b/lexers/testdata/README.md
deleted file mode 100644
index ec46906..0000000
--- a/lexers/testdata/README.md
+++ /dev/null
@@ -1,33 +0,0 @@
-# Lexer tests
-
-This directory contains input source and expected output lexer tokens.
-
-Input filenames for lexers are in the form `<name>.actual`. Expected output filenames are in the form `<name>.expected`.
-
-Each input filename is parsed by the corresponding lexer and checked against the expected JSON-encoded token list.
-
-
-To add/update tests do the following:
-
-1. `export LEXER=csharp`
-1. Create/edit a file `lexers/testdata/${LEXER}.actual` (eg. `csharp.actual`).
-2. Run `go run ./cmd/chroma/main.go --lexer ${LEXER} --json lexers/testdata/${LEXER}.actual > lexers/testdata/${LEXER}.expected`.
-3. Run `go test -v -run TestLexers ./lexers`.
-
-
-eg.
-
-```bash
-$ export LEXER=csharp
-$ go run ./cmd/chroma/main.go --lexer ${LEXER} --json lexers/testdata/${LEXER}.actual > lexers/testdata/${LEXER}.expected
-$ go test -v -run TestLexers ./lexers
-=== RUN   TestLexers
-=== RUN   TestLexers/C#
-=== RUN   TestLexers/CSS
---- PASS: TestLexers (0.01s)
-    --- PASS: TestLexers/C# (0.00s)
-    --- PASS: TestLexers/CSS (0.00s)
-PASS
-ok    github.com/alecthomas/chroma/lexers 0.032s
-```
-