diff --git a/assert.gs b/assert.gs
index 867d7a4..b378dd7 100644
--- a/assert.gs
+++ b/assert.gs
@@ -21,3 +21,16 @@ proc assert result, expected, message = "" {
     }    
 }
 %endif
+
+# assert true
+proc assert_t condition, message {
+    if not $condition {
+        error $message;
+    }
+}
+# assert false
+proc assert_f condition, message {
+    if $condition {
+        error $message;
+    }
+}
diff --git a/string.gs b/string.gs
index b6386d7..8510a9f 100644
--- a/string.gs
+++ b/string.gs
@@ -1,4 +1,16 @@
+%define WHITESPACE " \t\n\r"
+%define ASCII_LOWERCASE "abcdefghijklmnopqrstuvwxyz"
+%define ASCII_UPPERCASE "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+%define ASCII_LETTERS ASCII_LOWERCASE & ASCII_UPPERCASE
+%define DIGITS "0123456789"
+%define HEXDIGITS DIGITS & "abcdef" & "ABCDEF"
+%define OCTDIGITS "01234567"
+%define PUNCTUATION "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
+%define PRINTABLE_CHARS DIGITS & ASCII_LETTERS & PUNCTUATION & WHITESPACE
+
 func strcmp(string1, string2) {
+    # This code assumes you have costumes A-Z and a-z
+    # If you need to compare chars e.g ä vs Ä, then you need to add those as costumes too
     if length($string1) != length($string2) {
         return false;
     }
@@ -14,3 +26,119 @@ func strcmp(string1, string2) {
     }
     return true;
 }
+
+func slice(string, start, end) {
+    local ret = "";
+    local i = $start;
+
+    until i >= $end {
+        ret &= $string[i];
+        i++;
+    }
+    return ret;
+}
+
+func slice_step(string, start, end, step) {
+    if $start == $end or $step + "" == 0 {
+        return "";
+    } else {
+        local ret = "";
+        local i = $start;
+
+        if $step < 0 {
+            until i <= $end {
+                ret &= $string[i];
+                i += $step;
+            }
+            return ret;
+        } else {
+            until i >= $end {
+                ret &= $string[i];
+                i += $step;
+            }
+            return ret;
+        }
+    }
+}
+
+func startswith(string, start) {
+    local i = 1;
+    repeat length $start {
+        if $string[i] != $start[i] {
+            return false;
+        }
+        i++;
+    }
+    return true;
+}
+
+# startswith_from(1, ...) is equivalent to startswith(...)
+func startswith_from(i, string, start) {
+    local i = 1;
+    repeat length $start {
+        if $string[i + $i - 1] != $start[i] {
+            return false;
+        }
+        i++;
+    }
+    return true;
+}
+
+func endswith(string, end) {
+    local i = 0;
+    repeat length $end {
+        if $string[length $string - i] != $end[length $end - i] {
+            return false;
+        }
+        i++;
+    }
+    return true;
+}
+
+# endswith_from(length <str>, <str>, ...) is equivalent to endswith(<str>, ...)
+func endswith_from(i, string, end) {
+    local i = 0;
+    repeat length $end {
+        if $string[$i - i] != $end[length $end - i] {
+            return false;
+        }
+        i++;
+    }
+    return true;
+}
+
+# Pad '0' to the left until it reaches the specified length.
+func zfill(string, len) {
+    local ret = $string;
+    repeat $len - length $string {
+        ret = 0 & ret;
+    }
+    return ret;
+}
+
+# Pad ' ' to the left until it reaches the specified length.
+func ljust(string, len, char = " ") {
+    local ret = $string;
+    repeat $len - length $string {
+        ret = $char & ret;
+    }
+    return ret;
+}
+
+# Pad ' ' to the right until it reaches the specified length.
+func rjust(string, len, char = " ") {
+    local ret = $string;
+    repeat $len - length $string {
+        ret &= $char;
+    }
+    return ret;
+}
+
+# repeat $string $times times (equivalent to int-string multiplication in python)
+func repstr(string, times) {
+    local ret = "";
+    repeat $times {
+        ret &= $string;
+    }
+    return ret;
+}
diff --git a/test/lib/test_string.gs b/test/lib/test_string.gs
index 6542980..c316001 100644
--- a/test/lib/test_string.gs
+++ b/test/lib/test_string.gs
@@ -1,8 +1,27 @@
 proc test_string {
-    if not strcmp("Hello, World!", "Hello, World!") == true {
-        error "strcmp(\"Hello, World!\", \"Hello, World!\")";
-    }
-    if not strcmp("hello, world!", "Hello, World!") == false {
-        error "strcmp(\"hello, world!\", \"Hello, World!\")";
-    }
+    assert_t strcmp("Hello, World!", "Hello, World!"), "strcmp(\"Hello, World!\", \"Hello, World!\")";
+    assert_f strcmp("hello, world!", "Hello, World!"), "strcmp(\"hello, world!\", \"Hello, World!\")";
+
+    assert_t startswith("Hello, World!", "Hello"), "startswith(\"Hello, World!\", \"Hello\")";
+    assert_t startswith_from(8, "Hello, World!", "World!"), "startswith_from(8, \"Hello, World!\", \"World!\")";
+    assert_t endswith("Hello, World!", "World!"), "endswith(\"Hello, World!\", \"World!\")";
+    assert_t endswith_from(6, "Hello, World!", "Hello,"), "endswith_from(6, \"Hello, World!\", \"Hello,\")";
+    assert zfill("FFF", 6), "000FFF", "zfill(\"FFF\", 6)";
+    assert ljust("FFF", 6), "   FFF", "ljust(\"FFF\", 6)";
+    assert ljust("FFF", 6, 1), "111FFF", "ljust(\"FFF\", 6, 1)";
+    assert repstr("a", 3), "aaa", "repstr(\"a\", 3)";
+    assert rjust("FFF", 6), "FFF   ", "rjust(\"FFF\", 6)";
+    assert rjust("FFF", 6, 0), "FFF000", "rjust(\"FFF\", 6, 1)";
+    assert slice("Hello, world", 2, 6), "ello", "slice(\"Hello, world\", 2, 6)";
+    assert slice_step("Hello, world", 5, 0, -2), "olH", "slice_step(\"Hello, world\", 5, 0, -2)";
+
+    assert WHITESPACE, " \t\n\r", "WHITESPACE";
+    assert ASCII_LETTERS, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ", "ASCII_LETTERS";
+    assert ASCII_LOWERCASE, "abcdefghijklmnopqrstuvwxyz", "ASCII_LOWERCASE";
+    assert ASCII_UPPERCASE, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "ASCII_UPPERCASE";
+    assert DIGITS, "0123456789", "DIGITS";
+    assert HEXDIGITS, "0123456789abcdefABCDEF", "HEXDIGITS";
+    assert OCTDIGITS, "01234567", "OCTDIGITS";
+    assert PUNCTUATION, "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~", "PUNCTUATION";
+    assert PRINTABLE_CHARS, "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~ \t\n\r", "PRINTABLE_CHARS";
 }