diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index e331a21..d6c7d3f 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -29,6 +29,9 @@ add_library(apfl
 add_executable(apfl-bin main.c)
 target_link_libraries(apfl-bin PUBLIC apfl)
 
+add_executable(functional-test-runner functional-test-runner.c)
+target_link_libraries(functional-test-runner PUBLIC apfl)
+
 set_target_properties(apfl-bin
     PROPERTIES RUNTIME_OUTPUT_NAME apfl
 )
@@ -43,6 +46,13 @@ unittest(tokenizer_test "")
 unittest(parser_test "")
 unittest(resizable_test "resizable.h")
 unittest(hashmap_test "hashmap.h")
+unittest(strings_test "")
+
+function(functionaltest name)
+    add_test(NAME "functionaltest_${name}" COMMAND functional-test-runner ${CMAKE_SOURCE_DIR}/src/functional-tests/${name}.at)
+endfunction()
+
+functionaltest("hello-world")
 
 install(TARGETS apfl DESTINATION lib)
 install(TARGETS apfl-bin DESTINATION bin)
diff --git a/src/apfl.h b/src/apfl.h
index 4e359c9..71e7d1f 100644
--- a/src/apfl.h
+++ b/src/apfl.h
@@ -110,10 +110,21 @@ void apfl_string_builder_init(struct apfl_allocator allocator, struct apfl_strin
 void apfl_string_builder_deinit(struct apfl_string_builder *);
 bool apfl_string_builder_append(struct apfl_string_builder *, struct apfl_string_view);
 bool apfl_string_builder_append_byte(struct apfl_string_builder *, char byte);
+bool apfl_string_builder_append_bytes(struct apfl_string_builder *, const char *bytes, size_t len);
 struct apfl_string apfl_string_builder_move_string(struct apfl_string_builder *);
 
 #define apfl_string_builder_append_cstr(builder, cstr) (apfl_string_builder_append((builder), apfl_string_view_from_cstr((cstr))))
 
+struct apfl_string_view apfl_string_view_offset(struct apfl_string_view sv, size_t off);
+struct apfl_string_view apfl_string_view_trunc(struct apfl_string_view sv, size_t newlen);
+struct apfl_string_view apfl_string_view_substr(struct apfl_string_view sv, size_t off, size_t newlen);
+
+ptrdiff_t apfl_string_view_search(struct apfl_string_view haystack, struct apfl_string_view needle);
+
+struct apfl_string_view apfl_string_view_ltrim(struct apfl_string_view sv);
+struct apfl_string_view apfl_string_view_rtrim(struct apfl_string_view sv);
+struct apfl_string_view apfl_string_view_trim(struct apfl_string_view sv);
+
 struct apfl_format_writer {
     bool (*write)(void *, const char *buf, size_t len);
     void *opaque;
diff --git a/src/functional-test-runner.c b/src/functional-test-runner.c
new file mode 100644
index 0000000..a7caa21
--- /dev/null
+++ b/src/functional-test-runner.c
@@ -0,0 +1,243 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+#include <setjmp.h>
+#include <stdarg.h>
+
+#include "apfl.h"
+
+#include "alloc.h"
+
+enum testresult {
+    T_OK,
+    T_ERR,
+    T_FATAL,
+};
+
+struct alloc_context {
+    struct apfl_allocator wrapped;
+    jmp_buf *jmp;
+};
+
+struct test_parts {
+    struct apfl_string_view script;
+    struct apfl_string_view output;
+};
+
+APFL_NORETURN static void
+fatal(jmp_buf *jmp, const char *fmt, ...)
+{
+    va_list varargs;
+    va_start(varargs, fmt);
+    vfprintf(stderr, fmt, varargs);
+    va_end(varargs);
+
+    longjmp(*jmp, 1);
+}
+
+static void *
+alloc_cb(void *opaque, void *oldptr, size_t oldsize, size_t newsize)
+{
+    struct alloc_context *alloc_ctx = opaque;
+
+    void *out = ALLOCATOR_CALL(alloc_ctx->wrapped, oldptr, oldsize, newsize);
+
+    if (newsize != 0 && out == NULL) {
+        fatal(
+            alloc_ctx->jmp,
+            "Failed to allocate: oldptr=%p, oldsize=%lld, newsize=%lld\n",
+            oldptr,
+            (long long int)oldsize,
+            (long long int)newsize
+        );
+    }
+
+    return out;
+}
+
+#define BUFSIZE 4096
+
+static bool
+file_get_contents(const char *filename, struct apfl_string *str, struct apfl_allocator allocator)
+{
+    FILE *f = fopen(filename, "r");
+    if (f == NULL) {
+        return false;
+    }
+
+    struct apfl_string_builder sb;
+    apfl_string_builder_init(allocator, &sb);
+
+    char buf[BUFSIZE];
+    while (!feof(f)) {
+        size_t len = fread(buf, 1, BUFSIZE, f);
+        if (!apfl_string_builder_append_bytes(&sb, buf, len)) {
+            apfl_string_builder_deinit(&sb);
+            fclose(f);
+            return false;
+        }
+    }
+
+    if (!feof(f)) {
+        apfl_string_builder_deinit(&sb);
+        fclose(f);
+        return false;
+    }
+
+    fclose(f);
+
+    *str = apfl_string_builder_move_string(&sb);
+    apfl_string_builder_deinit(&sb);
+
+    return true;
+}
+
+bool
+parse_test(const struct apfl_string str, struct test_parts *parts)
+{
+    struct apfl_string_view sv = apfl_string_view_from(str);
+
+    struct apfl_string_view marker_script = apfl_string_view_from("===== script =====\n");
+    struct apfl_string_view marker_output = apfl_string_view_from("===== output =====\n");
+
+    ptrdiff_t script = apfl_string_view_search(sv, marker_script);
+    ptrdiff_t output = apfl_string_view_search(sv, marker_output);
+
+    if (script < 0 || output < 0 || script > output) {
+        return false;
+    }
+
+    parts->script = apfl_string_view_substr(
+        sv,
+        script + marker_script.len,
+        output - script - marker_script.len
+    );
+    parts->output = apfl_string_view_offset(sv, output + marker_output.len);
+
+    return true;
+}
+
+static void
+dump_stack_error(apfl_ctx ctx, apfl_iterative_runner runner)
+{
+    if (apfl_iterative_runner_has_error_on_stack(runner)) {
+        assert(apfl_debug_print_val(ctx, -1, apfl_format_file_writer(stderr)));
+    }
+}
+
+enum testresult
+runtest(const char *filename)
+{
+    jmp_buf jmp;
+
+    struct alloc_context alloc_ctx = {
+        .wrapped = apfl_allocator_default(),
+        .jmp = &jmp,
+    };
+
+    struct apfl_allocator allocator = {
+        .opaque = &alloc_ctx,
+        .alloc = alloc_cb,
+    };
+
+    if (setjmp(jmp) != 0) {
+        return T_FATAL;
+    }
+
+    struct apfl_string content;
+    if (!file_get_contents(filename, &content, allocator)) {
+        fprintf(stderr, "Could not read file \"%s\"\n", filename);
+        return T_ERR;
+    }
+
+    struct test_parts parts;
+    if (!parse_test(content, &parts)) {
+        fprintf(stderr, "Could not parse test \"%s\"\n", filename);
+        return T_ERR;
+    }
+
+    struct apfl_string_builder output;
+    apfl_string_builder_init(allocator, &output);
+
+    apfl_ctx ctx = apfl_ctx_new((struct apfl_config) {
+        .allocator = allocator,
+        .output_writer = apfl_format_string_writer(&output),
+    });
+
+    struct apfl_string_source_reader_data src_data = apfl_string_source_reader_create(parts.script);
+    apfl_iterative_runner runner = apfl_iterative_runner_new(ctx, apfl_string_source_reader(&src_data));
+    assert(runner != NULL);
+
+    while (apfl_iterative_runner_next(runner)) {
+        switch (apfl_iterative_runner_get_result(runner)) {
+        case APFL_RESULT_OK :
+            break;
+        case APFL_RESULT_ERR:
+            fprintf(stderr, "Error occurred during evaluation.\n");
+            dump_stack_error(ctx, runner);
+            return T_ERR;
+        case APFL_RESULT_ERR_FATAL:
+            fprintf(stderr, "Fatal error occurred during evaluation.\n");
+            dump_stack_error(ctx, runner);
+            return T_FATAL;
+        }
+    }
+
+    apfl_iterative_runner_destroy(runner);
+    apfl_ctx_destroy(ctx);
+
+    struct apfl_string output_string = apfl_string_builder_move_string(&output);
+    struct apfl_string_view have = apfl_string_view_trim(apfl_string_view_from(output_string));
+    struct apfl_string_view want = apfl_string_view_trim(parts.output);
+
+    if (!apfl_string_eq(have, want)) {
+        fprintf(
+            stderr,
+
+            "Test failed\n"
+            "=== WANT ===\n"
+            APFL_STR_FMT "\n"
+            "=== HAVE ===\n"
+            APFL_STR_FMT "\n",
+
+            APFL_STR_FMT_ARGS(want),
+            APFL_STR_FMT_ARGS(have)
+        );
+
+        apfl_string_deinit(allocator, &output_string);
+        apfl_string_deinit(allocator, &content);
+
+        return T_ERR;
+    }
+
+    apfl_string_deinit(allocator, &output_string);
+    apfl_string_deinit(allocator, &content);
+
+    return T_OK;
+}
+
+int
+main(int argc, const char **argv)
+{
+    if (argc < 2) {
+        fprintf(stderr, "%s: Need at least one argument\n", argv[0]);
+        return 1;
+    }
+
+    bool ok = true;
+
+    for (int i = 1; i < argc; i++) {
+        switch (runtest(argv[i])) {
+        case T_OK:
+            break;
+        case T_ERR:
+            ok = false;
+            break;
+        case T_FATAL:
+            return 1;
+        }
+    }
+
+    return ok ? 0 : 1;
+}
diff --git a/src/functional-tests/hello-world.at b/src/functional-tests/hello-world.at
new file mode 100644
index 0000000..7100dd4
--- /dev/null
+++ b/src/functional-tests/hello-world.at
@@ -0,0 +1,5 @@
+===== script =====
+print "Hello World!"
+
+===== output =====
+Hello World!
diff --git a/src/strings.c b/src/strings.c
index 4131174..36a6c41 100644
--- a/src/strings.c
+++ b/src/strings.c
@@ -1,5 +1,7 @@
+#include <ctype.h>
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -117,8 +119,8 @@ apfl_string_builder_deinit(struct apfl_string_builder *builder)
     builder_reset(builder);
 }
 
-static bool
-append_bytes(struct apfl_string_builder *builder, const char *bytes, size_t len)
+bool
+apfl_string_builder_append_bytes(struct apfl_string_builder *builder, const char *bytes, size_t len)
 {
     return apfl_resizable_append(
         builder->allocator,
@@ -132,13 +134,13 @@ append_bytes(struct apfl_string_builder *builder, const char *bytes, size_t len)
 bool
 apfl_string_builder_append(struct apfl_string_builder *builder, struct apfl_string_view view)
 {
-    return append_bytes(builder, view.bytes, view.len);
+    return apfl_string_builder_append_bytes(builder, view.bytes, view.len);
 }
 
 bool
 apfl_string_builder_append_byte(struct apfl_string_builder *builder, char byte)
 {
-    return append_bytes(builder, &byte, 1);
+    return apfl_string_builder_append_bytes(builder, &byte, 1);
 }
 
 struct apfl_string
@@ -166,3 +168,88 @@ apfl_string_move_into_new_gc_string(struct gc *gc, struct apfl_string *in)
     *str = apfl_string_move(in);
     return str;
 }
+
+struct apfl_string_view
+apfl_string_view_offset(struct apfl_string_view sv, size_t off)
+{
+    if (off >= sv.len) {
+        return (struct apfl_string_view) {
+            .bytes = NULL,
+            .len = 0,
+        };
+    }
+
+    sv.bytes += off;
+    sv.len -= off;
+
+    return sv;
+}
+
+struct apfl_string_view
+apfl_string_view_trunc(struct apfl_string_view sv, size_t newlen)
+{
+    if (newlen < sv.len) {
+        sv.len = newlen;
+    }
+    return sv;
+}
+
+struct apfl_string_view
+apfl_string_view_substr(struct apfl_string_view sv, size_t off, size_t newlen)
+{
+    sv = apfl_string_view_offset(sv, off);
+    sv = apfl_string_view_trunc(sv, newlen);
+    return sv;
+}
+
+ptrdiff_t
+apfl_string_view_search(const struct apfl_string_view haystack, const struct apfl_string_view needle)
+{
+    // TODO: This is not very efficient. Use a faster algorithm here.
+
+    if (needle.len == 0) {
+        return 0;
+    }
+
+    if (needle.len > haystack.len || haystack.len == 0) {
+        return -1;
+    }
+
+    size_t lim = haystack.len - needle.len;
+    if (lim > PTRDIFF_MAX) {
+        return -1;
+    }
+
+    for (size_t i = 0; i <= lim; i++) {
+        if (memcmp(haystack.bytes + i, needle.bytes, needle.len) == 0) {
+            return (ptrdiff_t)i;
+        }
+    }
+
+    return -1;
+}
+
+struct apfl_string_view
+apfl_string_view_ltrim(struct apfl_string_view sv)
+{
+    while (sv.len > 0 && isspace(sv.bytes[0])) {
+        sv.len--;
+        sv.bytes++;
+    }
+    return sv;
+}
+
+struct apfl_string_view
+apfl_string_view_rtrim(struct apfl_string_view sv)
+{
+    while (sv.len > 0 && isspace(sv.bytes[sv.len - 1])) {
+        sv.len--;
+    }
+    return sv;
+}
+
+struct apfl_string_view
+apfl_string_view_trim(struct apfl_string_view sv)
+{
+    return apfl_string_view_ltrim(apfl_string_view_rtrim(sv));
+}
diff --git a/src/strings_test.c b/src/strings_test.c
new file mode 100644
index 0000000..4fd5115
--- /dev/null
+++ b/src/strings_test.c
@@ -0,0 +1,164 @@
+#include "apfl.h"
+#include "test.h"
+
+static void
+search_testcase(testctx t, const char *haystack, const char *needle, ptrdiff_t want)
+{
+    ptrdiff_t have = apfl_string_view_search(
+        apfl_string_view_from(haystack),
+        apfl_string_view_from(needle)
+    );
+
+    if (have != want) {
+        test_failf(t, "search \"%s\" in \"%s\" failed: want=%d, have=%d", needle, haystack, (int)want, (int)have);
+    }
+}
+
+TEST(search, t) {
+    search_testcase(t, "", "", 0);
+    search_testcase(t, "", "foo", -1);
+    search_testcase(t, "foo", "foo", 0);
+    search_testcase(t, "foofoo", "foo", 0);
+    search_testcase(t, "hoofoo", "foo", 3);
+    search_testcase(t, "hoofooo", "foo", 3);
+    search_testcase(t, "ababab", "a", 0);
+    search_testcase(t, "ababab", "b", 1);
+    search_testcase(t, "ababab", "ab", 0);
+    search_testcase(t, "ababab", "ababab", 0);
+}
+
+typedef struct apfl_string_view (*trimmer)(struct apfl_string_view);
+
+static void
+generic_trim_testcase(testctx t, const char *name, trimmer trim, const char *in, const char *want)
+{
+    struct apfl_string_view have = trim(apfl_string_view_from(in));
+    if (!apfl_string_eq(want, have)) {
+        test_failf(t, "%s of \"%s\" failed: want=\"%s\", have=\"" APFL_STR_FMT "\"", name, in, want, APFL_STR_FMT_ARGS(have));
+    }
+}
+
+static void
+ltrim_testcase(testctx t, const char *in, const char *want)
+{
+    generic_trim_testcase(t, "ltrim", apfl_string_view_ltrim, in, want);
+}
+
+static void
+rtrim_testcase(testctx t, const char *in, const char *want)
+{
+    generic_trim_testcase(t, "rtrim", apfl_string_view_rtrim, in, want);
+}
+
+static void
+trim_testcase(testctx t, const char *in, const char *want)
+{
+    generic_trim_testcase(t, "trim", apfl_string_view_trim, in, want);
+}
+
+TEST(ltrim, t) {
+    ltrim_testcase(t, "", "");
+    ltrim_testcase(t, " ", "");
+    ltrim_testcase(t, "  ", "");
+    ltrim_testcase(t, " \n", "");
+    ltrim_testcase(t, " \t\n", "");
+    ltrim_testcase(t, " foo", "foo");
+    ltrim_testcase(t, " foo ", "foo ");
+    ltrim_testcase(t, "foo ", "foo ");
+    ltrim_testcase(t, "foo\t", "foo\t");
+    ltrim_testcase(t, " f o o ", "f o o ");
+}
+
+TEST(rtrim, t) {
+    rtrim_testcase(t, "", "");
+    rtrim_testcase(t, " ", "");
+    rtrim_testcase(t, "  ", "");
+    rtrim_testcase(t, " \n", "");
+    rtrim_testcase(t, " \t\n", "");
+    rtrim_testcase(t, " foo", " foo");
+    rtrim_testcase(t, " foo ", " foo");
+    rtrim_testcase(t, "foo ", "foo");
+    rtrim_testcase(t, "foo\t", "foo");
+    rtrim_testcase(t, " f o o ", " f o o");
+}
+
+TEST(trim, t) {
+    trim_testcase(t, "", "");
+    trim_testcase(t, " ", "");
+    trim_testcase(t, "  ", "");
+    trim_testcase(t, " \n", "");
+    trim_testcase(t, " \t\n", "");
+    trim_testcase(t, " foo", "foo");
+    trim_testcase(t, " foo ", "foo");
+    trim_testcase(t, "foo ", "foo");
+    trim_testcase(t, "foo\t", "foo");
+    trim_testcase(t, " f o o ", "f o o");
+}
+
+static void
+offset_testcase(testctx t, const char *in, size_t off, const char *want)
+{
+    struct apfl_string_view have = apfl_string_view_offset(apfl_string_view_from(in), off);
+    if (!apfl_string_eq(want, have)) {
+        test_failf(t, "offset(\"%s\", %d) failed: want=\"%s\", have=\"" APFL_STR_FMT "\"", in, (int)off, want, APFL_STR_FMT_ARGS(have));
+    }
+}
+
+TEST(offset, t) {
+    offset_testcase(t, "", 0, "");
+    offset_testcase(t, "", 10, "");
+    offset_testcase(t, "foobar", 0, "foobar");
+    offset_testcase(t, "foobar", 1, "oobar");
+    offset_testcase(t, "foobar", 3, "bar");
+    offset_testcase(t, "foobar", 6, "");
+    offset_testcase(t, "foobar", 1000, "");
+}
+
+static void
+trunc_testcase(testctx t, const char *in, size_t newlen, const char *want)
+{
+    struct apfl_string_view have = apfl_string_view_trunc(apfl_string_view_from(in), newlen);
+    if (!apfl_string_eq(want, have)) {
+        test_failf(t, "trunc(\"%s\", %d) failed: want=\"%s\", have=\"" APFL_STR_FMT "\"", in, (int)newlen, want, APFL_STR_FMT_ARGS(have));
+    }
+}
+
+TEST(trunc, t) {
+    trunc_testcase(t, "", 0, "");
+    trunc_testcase(t, "", 10, "");
+    trunc_testcase(t, "foobar", 6, "foobar");
+    trunc_testcase(t, "foobar", 10000, "foobar");
+    trunc_testcase(t, "foobar", 0, "");
+    trunc_testcase(t, "foobar", 5, "fooba");
+    trunc_testcase(t, "foobar", 3, "foo");
+}
+
+static void
+substr_testcase(testctx t, const char *in, size_t off, size_t newlen, const char *want)
+{
+    struct apfl_string_view have = apfl_string_view_substr(apfl_string_view_from(in), off, newlen);
+    if (!apfl_string_eq(want, have)) {
+        test_failf(t, "substr(\"%s\", %d, %d) failed: want=\"%s\", have=\"" APFL_STR_FMT "\"", in, (int)off, (int)newlen, want, APFL_STR_FMT_ARGS(have));
+    }
+}
+
+TEST(substr, t) {
+    substr_testcase(t, "", 0, 0, "");
+    substr_testcase(t, "", 10, 0, "");
+    substr_testcase(t, "", 0, 10, "");
+    substr_testcase(t, "foobar", 0, 10, "foobar");
+    substr_testcase(t, "foobar", 0, 6, "foobar");
+    substr_testcase(t, "foobar", 0, 5, "fooba");
+    substr_testcase(t, "foobar", 1, 5, "oobar");
+    substr_testcase(t, "foobar", 100, 5, "");
+}
+
+TESTS_BEGIN
+    ADDTEST(search),
+    ADDTEST(ltrim),
+    ADDTEST(rtrim),
+    ADDTEST(trim),
+    ADDTEST(offset),
+    ADDTEST(trunc),
+    ADDTEST(substr),
+TESTS_END