diff --git a/lexer/src/tests.rs b/lexer/src/tests.rs
index b36f74b..b6ed4f3 100644
--- a/lexer/src/tests.rs
+++ b/lexer/src/tests.rs
@@ -1 +1,218 @@
-// Placeholder: real tests to be added.
+// This file is part of the uutils awk package.
+//
+// For the full copyright and license information, please view the LICENSE
+// files that was distributed with this source code.
+
+use std::io::Write;
+
+use crate::{Context, Extra, Identifier, Lexer, Token};
+use bumpalo::{
+    Bump,
+    collections::{CollectIn, Vec},
+};
+
+fn lex<'a>(
+    src: &'a [u8],
+    arena: &'a Bump,
+    posix_strict: bool,
+    gnu_strict: bool,
+) -> Vec<'a, Token<'a>> {
+    Lexer::with_extras(
+        src,
+        Extra {
+            ctx: Context::AcceptExpression,
+            arena,
+            posix_strict,
+            gnu_strict,
+        },
+    )
+    .collect_in::<Result<Vec<_>, _>>(arena)
+    .unwrap()
+}
+
+#[test]
+fn lexer_test_newlines_non_posix() {
+    let mixed = " \t \n \t\n\n \\\n \t";
+    let arena = Bump::new();
+    let mut str = Vec::new_in(&arena);
+    for tok in ["BEGIN", "{", "else", "do", "&&", "||", "?", ":", ","] {
+        write!(str, "{tok}{mixed}").unwrap();
+    }
+    str.push(b'}');
+    assert_eq!(
+        &lex(&str, &arena, false, false),
+        &[
+            Token::BeginPattern,
+            Token::Newline,
+            Token::Newline,
+            Token::OpenBrace,
+            Token::Else,
+            Token::Do,
+            Token::BooleanAnd,
+            Token::BooleanOr,
+            Token::QuestionMark,
+            Token::Colon,
+            Token::Comma,
+            Token::ClosedBrace
+        ]
+    );
+}
+
+#[test]
+#[should_panic]
+fn lexer_test_newlines_posix() {
+    let mixed = " \t \n \t\n\n \\\n \t";
+    let arena = Bump::new();
+    let mut str = Vec::new_in(&arena);
+    for tok in ["BEGIN", "{", "else", "do", "&&", "||", "?", ":", ","] {
+        write!(str, "{tok}{mixed}").unwrap();
+    }
+    str.push(b'}');
+    lex(&str, &arena, true, false);
+}
+
+#[test]
+fn lexer_test_collapsible_delimiters() {
+    let arena = Bump::new();
+    let str = b";\\\n;;;\n\n\n\n;;;\n\\\n\n";
+    assert_eq!(
+        &lex(str, &arena, false, false),
+        &[
+            Token::Semicolon,
+            Token::Semicolon,
+            Token::Newline,
+            Token::Semicolon,
+            Token::Newline,
+            Token::Newline,
+        ]
+    );
+}
+
+#[test]
+fn lexer_test_multiline() {
+    let arena = Bump::new();
+    let str = b"\"aaaa\\\nbbbb\", /ccc\\\nd/";
+    assert_eq!(
+        &lex(str, &arena, false, false),
+        &[
+            Token::String(b"aaaabbbb".into()),
+            Token::Comma,
+            Token::Regex(b"cccd".into())
+        ]
+    );
+}
+
+#[test]
+#[should_panic]
+fn lexer_test_uu_extensions() {
+    let arena = Bump::new();
+    lex(b"@concurrent", &arena, false, true);
+}
+
+#[test]
+fn lexer_test_gnu_pattern() {
+    let arena = Bump::new();
+    assert_eq!(
+        &lex(b"BEGINFILE ENDFILE", &arena, true, false),
+        &[
+            Token::Identifier(Identifier {
+                namespace: None,
+                literal: "BEGINFILE"
+            }),
+            Token::Identifier(Identifier {
+                namespace: None,
+                literal: "ENDFILE"
+            })
+        ]
+    );
+}
+
+#[test]
+fn lexer_test_floats() {
+    let arena = Bump::new();
+    let str = b"1 20. 0. .3 2e4 -3.e2 5e+1 2.1e-3";
+    assert_eq!(
+        &lex(str, &arena, false, false),
+        &[
+            Token::Number(1.),
+            Token::Number(20.),
+            Token::Number(0.),
+            Token::Number(0.3),
+            Token::Number(2e4),
+            Token::Number(-3e2),
+            Token::Number(5e1),
+            Token::Number(2.1e-3)
+        ]
+    );
+}
+
+#[test]
+fn lexer_test_directive_escaping() {
+    let arena = Bump::new();
+    let str = br#" @include "aa\"a\ta" @nsinclude "b\"\nb" "#;
+    assert_eq!(
+        &lex(str, &arena, false, false),
+        &[
+            Token::IncludeDirective(b"aa\"a\ta".into()),
+            Token::NsIncludeDirective(b"b\"\nb".into())
+        ]
+    );
+}
+
+#[test]
+#[should_panic]
+fn lexer_test_ident_rules_non_posix() {
+    let arena = Bump::new();
+    lex(b"@namespace \"1a\"; a::1a", &arena, false, false);
+}
+
+#[test]
+#[should_panic]
+fn lexer_test_ident_rules_posix() {
+    let arena = Bump::new();
+    lex(b"@namespace \"foo\"; foo::a", &arena, true, false);
+}
+
+#[test]
+fn lexer_test_general_tokens() {
+    let arena = Bump::new();
+    let str = br#"
+        @load "lib1.so.1"
+        BEGIN { print a + 1 }
+        /2\..*/;
+        END { $1 == foo::bar }
+    "#;
+    assert_eq!(
+        &lex(str, &arena, false, false),
+        &[
+            Token::Newline,
+            Token::LoadDirective(b"lib1.so.1".into()),
+            Token::Newline,
+            Token::BeginPattern,
+            Token::OpenBrace,
+            Token::Print,
+            Token::Identifier(Identifier {
+                namespace: None,
+                literal: "a"
+            }),
+            Token::Plus,
+            Token::Number(1.),
+            Token::ClosedBrace,
+            Token::Newline,
+            Token::Regex(b"2\\..*".into()),
+            Token::Semicolon,
+            Token::Newline,
+            Token::EndPattern,
+            Token::OpenBrace,
+            Token::Record,
+            Token::Number(1.),
+            Token::EqualTo,
+            Token::Identifier(Identifier {
+                namespace: Some("foo"),
+                literal: "bar"
+            }),
+            Token::ClosedBrace,
+            Token::Newline
+        ]
+    );
+}