diff --git a/dice/__init__.py b/dice/__init__.py
index 96f30f3..7b1c49f 100644
--- a/dice/__init__.py
+++ b/dice/__init__.py
@@ -7,3 +7,9 @@ from dice import lib
 async def choose(*choices: str):
     """Randomly select one item from multiple choices."""
     await hitomi.say(lib.choose(*choices))
+
+
+@hitomi.command()
+async def roll(*, roll_str: str):
+    """Provided a dice string, roll the dice and return the result."""
+    await hitomi.say(lib.roll(roll_str))
diff --git a/dice/lib.py b/dice/lib.py
index 7406d24..efd8fa0 100644
--- a/dice/lib.py
+++ b/dice/lib.py
@@ -1,7 +1,271 @@
 """Functions for use in the dice module."""
 import random
 
+import ply.lex as lex
+import ply.yacc as yacc
+
+
+class DiceRoller(object):
+
+    tokens = ['NUMBER', 'TEXT', 'ROLLSEP']
+    literals = ['#', '/', '+', '-', 'd']
+
+    t_TEXT = r'\s+[^;]+'
+    t_ROLLSEP = r';\s*'
+
+    def build(self):
+        lex.lex(module=self)
+        yacc.yacc(module=self)
+
+    def t_NUMBER(self, t):
+        r'\d+'
+        t.value = int(t.value)
+        return t
+
+    def t_error(self, t):
+        t.lexer.skip(1)
+
+    precedence = (
+        ('left', 'ROLLSEP'),
+        ('left', '+', '-'),
+        ('right', 'd'),
+        ('left', '#'),
+        ('left', '/')
+    )
+
+    output = ""
+
+    def roll_dice(self, keep, dice, size):
+        """Takes the parsed dice string for a single roll (eg 3/4d20) and performs
+        the actual roll. Returns a string representing the result.
+        """
+
+        a = list(range(dice))
+        for i in range(dice):
+            a[i] = random.randint(1, size)
+        if keep != dice:
+            b = sorted(a, reverse=True)
+            b = b[0:keep]
+        else:
+            b = a
+        total = sum(b)
+        outstr = "[" + ",".join(str(i) for i in a) + "]"
+
+        return (total, outstr)
+
+    def process_roll(self, trials, mods, comment):
+        """Processes rolls coming from the parser.
+
+        This generates the inputs for the roll_dice() command, and returns
+        the full string representing the whole current dice string (the part
+        up to a semicolon or end of line).
+        """
+
+        output = ""
+        repeat = 1
+        if trials != None:
+            repeat = trials
+        for i in range(repeat):
+            mode = 1
+            total = 0
+            curr_str = ""
+            if i > 0:
+                output += ", "
+            for m in mods:
+                keep = 0
+                dice = 1
+                res = 0
+                # if m is a tuple, then it is a die roll
+                # m[0] = (keep, num dice)
+                # m[1] = num faces on the die
+                if type(m) == tuple:
+                    if m[0] != None:
+                        if m[0][0] != None:
+                            keep = m[0][0]
+                        dice = m[0][1]
+                    size = m[1]
+                    if keep > dice or keep == 0:
+                        keep = dice
+                    if size < 1:
+                        output = "# of sides for die is incorrect: %d" % size
+                        return output
+                    if dice < 1:
+                        output = "# of dice is incorrect: %d" % dice
+                        return output
+                    res = self.roll_dice(keep, dice, size)
+                    curr_str += "%d%s" % (res[0], res[1])
+                    res = res[0]
+                elif m == "+":
+                    mode = 1
+                    curr_str += "+"
+                elif m == "-":
+                    mode = -1
+                    curr_str += "-"
+                else:
+                    res = m
+                    curr_str += str(m)
+                total += mode * res
+            if repeat == 1:
+                if comment != None:
+                    output = "%d %s (%s)" % (total, comment.strip(), curr_str)
+                else:
+                    output = "%d (%s)" % (total, curr_str)
+            else:
+                output += "%d (%s)" % (total, curr_str)
+                if i == repeat - 1:
+                    if comment != None:
+                        output += " (%s)" % (comment.strip())
+        return output
+
+    def p_roll_r(self, p):
+        # Chain rolls together.
+
+        # General idea I had when creating this grammar: A roll string is a chain
+        # of modifiers, which may be repeated for a certain number of trials. It can
+        # have a comment that describes the roll
+        # Multiple roll strings can be chained with semicolon
+
+        'roll : roll ROLLSEP roll'
+        global output
+        p[0] = p[1] + "; " + p[3]
+        output = p[0]
+
+    def p_roll(self, p):
+        # Parse a basic roll string.
+
+        'roll : trial modifier comment'
+        global output
+        mods = []
+        if type(p[2]) == list:
+            mods = p[2]
+        else:
+            mods = [p[2]]
+        p[0] = self.process_roll(p[1], mods, p[3])
+        output = p[0]
+
+    def p_roll_no_trials(self, p):
+        # Parse a roll string without trials.
+
+        'roll : modifier comment'
+        global output
+        mods = []
+        if type(p[1]) == list:
+            mods = p[1]
+        else:
+            mods = [p[1]]
+        p[0] = self.process_roll(None, mods, p[2])
+        output = p[0]
+
+    def p_comment(self, p):
+        # Parse a comment.
+
+        '''comment : TEXT
+                   |'''
+        if len(p) == 2:
+            p[0] = p[1]
+        else:
+            p[0] = None
+
+    def p_modifier(self, p):
+        # Parse a modifier on a roll string.
+
+        '''modifier : modifier "+" modifier
+                    | modifier "-" modifier'''
+        # Use append to prevent nested lists (makes dealing with this easier)
+        if type(p[1]) == list:
+            p[1].append(p[2])
+            p[1].append(p[3])
+            p[0] = p[1]
+        elif type(p[3]) == list:
+            p[3].insert(0, p[2])
+            p[3].insert(0, p[1])
+            p[0] = p[3]
+        else:
+            p[0] = [p[1], p[2], p[3]]
+
+    def p_die(self, p):
+        # Return the left side before the "d", and the number of faces.
+
+        'modifier : left NUMBER'
+        p[0] = (p[1], p[2])
+
+    def p_die_num(self, p):
+        'modifier : NUMBER'
+        p[0] = p[1]
+
+    def p_left(self, p):
+        # Parse the number of dice we are rolling, and how many we are keeping.
+        'left : keep dice'
+        if p[1] == None:
+            p[0] = [None, p[2]]
+        else:
+            p[0] = [p[1], p[2]]
+
+    def p_left_all(self, p):
+        'left : dice'
+        p[0] = [None, p[1]]
+
+    def p_left_e(self, p):
+        'left :'
+        p[0] = None
+
+    def p_total(self, p):
+        'trial : NUMBER "#"'
+        if len(p) > 1:
+            p[0] = p[1]
+        else:
+            p[0] = None
+
+    def p_keep(self, p):
+        'keep : NUMBER "/"'
+        if p[1] != None:
+            p[0] = p[1]
+        else:
+            p[0] = None
+
+    def p_dice(self, p):
+        'dice : NUMBER "d"'
+        p[0] = p[1]
+
+    def p_dice_one(self, p):
+        'dice : "d"'
+        p[0] = 1
+
+    def p_error(self, p):
+        # Provide the user with something (albeit not much) when the roll can't be parsed.
+        global output
+        output = "Unable to parse roll"
+
+    def get_result(self):
+        global output
+        return output
+
+    def do_roll(self, dicestr):
+        """
+        Roll some dice and get the result (with broken out rolls).
+
+        Keyword arguments:
+        dicestr - format:
+        N#X/YdS+M label
+            N#: do the following roll N times (optional)
+            X/: take the top X rolls of the Y times rolled (optional)
+            Y : roll the die specified Y times (optional, defaults to 1)
+            dS: roll a S-sided die
+            +M: add M to the result (-M for subtraction) (optional)
+        """
+        self.build()
+        yacc.parse(dicestr)
+        return self.get_result()
+
+
+roller = DiceRoller()
+
 
 def choose(*choices: str):
     """Randomly choose an item from the provided multiple choices."""
     return random.choice(choices)
+
+
+def roll(roll_str: str):
+    """Parse a roll string with lex and yacc and return the result."""
+    return roller.do_roll(roll_str)