diff --git a/PFERD/transformer.py b/PFERD/transformer.py
new file mode 100644
index 0000000..1ecaf19
--- /dev/null
+++ b/PFERD/transformer.py
@@ -0,0 +1,238 @@
+import re
+from abc import ABC, abstractmethod
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Dict, Optional, Union
+
+
+class Rule(ABC):
+    @abstractmethod
+    def transform(self, path: Path) -> Optional[Path]:
+        pass
+
+
+class NormalRule(Rule):
+    def __init__(self, left: Path, right: Path):
+        self._left = left
+        self._right = right
+
+    def _match_prefix(self, path: Path) -> Optional[Path]:
+        left_parts = list(reversed(self._left.parts))
+        path_parts = list(reversed(path.parts))
+
+        if len(left_parts) > len(path_parts):
+            return None
+
+        while left_parts and path_parts:
+            left_part = left_parts.pop()
+            path_part = path_parts.pop()
+
+            if left_part != path_part:
+                return None
+
+        if left_parts:
+            return None
+
+        return Path(*path_parts)
+
+    def transform(self, path: Path) -> Optional[Path]:
+        if rest := self._match_prefix(path):
+            return self._right / rest
+
+        return None
+
+
+class ExactRule(Rule):
+    def __init__(self, left: Path, right: Path):
+        self._left = left
+        self._right = right
+
+    def transform(self, path: Path) -> Optional[Path]:
+        if path == self._left:
+            return self._right
+
+        return None
+
+
+class ReRule(Rule):
+    def __init__(self, left: str, right: str):
+        self._left = left
+        self._right = right
+
+    def transform(self, path: Path) -> Optional[Path]:
+        if match := re.fullmatch(self._left, str(path)):
+            kwargs: Dict[str, Union[int, float]] = {}
+
+            groups = [match[0]] + list(match.groups())
+            for i, group in enumerate(groups):
+                try:
+                    kwargs[f"i{i}"] = int(group)
+                except ValueError:
+                    pass
+
+                try:
+                    kwargs[f"f{i}"] = float(group)
+                except ValueError:
+                    pass
+
+            return Path(self._right.format(*groups, **kwargs))
+
+        return None
+
+
+@dataclass
+class RuleParseException(Exception):
+    line: "Line"
+    reason: str
+
+    def pretty_print(self) -> None:
+        print(f"Error parsing rule on line {self.line.line_nr}:")
+        print(self.line.line)
+        spaces = " " * self.line.index
+        print(f"{spaces}^--- {self.reason}")
+
+
+class Line:
+    def __init__(self, line: str, line_nr: int):
+        self._line = line
+        self._line_nr = line_nr
+        self._index = 0
+
+    def get(self) -> Optional[str]:
+        if self._index < len(self._line):
+            return self._line[self._index]
+
+        return None
+
+    @property
+    def line(self) -> str:
+        return self._line
+
+    @property
+    def line_nr(self) -> str:
+        return self._line
+
+    @property
+    def index(self) -> int:
+        return self._index
+
+    @index.setter
+    def index(self, index: int) -> None:
+        self._index = index
+
+    def advance(self) -> None:
+        self._index += 1
+
+    def expect(self, string: str) -> None:
+        for char in string:
+            if self.get() == char:
+                self.advance()
+            else:
+                raise RuleParseException(self, f"Expected {char!r}")
+
+
+QUOTATION_MARKS = {'"', "'"}
+
+
+def parse_string_literal(line: Line) -> str:
+    escaped = False
+    result = []
+
+    quotation_mark = line.get()
+    if quotation_mark not in QUOTATION_MARKS:
+        # This should never happen as long as this function is only called from
+        # parse_string.
+        raise RuleParseException(line, "Invalid quotation mark")
+    line.advance()
+
+    while c := line.get():
+        if escaped:
+            result.append(c)
+            escaped = False
+            line.advance()
+        elif c == quotation_mark:
+            line.advance()
+            return "".join(result)
+        elif c == "\\":
+            escaped = True
+            line.advance()
+        else:
+            result.append(c)
+            line.advance()
+
+    raise RuleParseException(line, "Expected end of string literal")
+
+
+def parse_until_space_or_eol(line: Line) -> str:
+    result = []
+    while c := line.get():
+        if c == " ":
+            break
+        result.append(c)
+        line.advance()
+
+    return "".join(result)
+
+
+def parse_string(line: Line) -> str:
+    if line.get() in QUOTATION_MARKS:
+        return parse_string_literal(line)
+    else:
+        return parse_until_space_or_eol(line)
+
+
+def parse_arrow(line: Line) -> str:
+    line.expect("-")
+
+    name = []
+    while True:
+        if c := line.get():
+            if c == "-":
+                break
+            else:
+                name.append(c)
+            line.advance()
+        else:
+            raise RuleParseException(line, "Expected rest of arrow")
+
+    line.expect("->")
+    return "".join(name)
+
+
+def parse_rule(line: Line) -> Rule:
+    left = parse_string(line)
+    line.expect(" ")
+    arrowindex = line.index
+    arrowname = parse_arrow(line)
+    line.expect(" ")
+    right = parse_string(line)
+
+    if arrowname == "":
+        return NormalRule(Path(left), Path(right))
+    elif arrowname == "exact":
+        return ExactRule(Path(left), Path(right))
+    elif arrowname == "re":
+        return ReRule(left, right)
+    else:
+        line.index = arrowindex + 1  # For nicer error message
+        raise RuleParseException(line, "Invalid arrow name")
+
+
+class Transformer:
+    def __init__(self, rules: str):
+        """
+        May throw a RuleParseException.
+        """
+
+        self._rules = []
+        for i, line in enumerate(rules.split("\n")):
+            line = line.strip()
+            if line:
+                self._rules.append(parse_rule(Line(line, i)))
+
+    def transform(self, path: Path) -> Optional[Path]:
+        for rule in self._rules:
+            if result := rule.transform(path):
+                return result
+
+        return None