diff --git a/renamebycsv/cli.py b/renamebycsv/cli.py
index 31e3351..37bdde5 100755
--- a/renamebycsv/cli.py
+++ b/renamebycsv/cli.py
@@ -1,88 +1,14 @@
 #!/usr/bin/env python3
 
 import argparse
-import csv
-import os
-import re
-from typing import Iterable
 import logging
 
-
-def find_all_candidates(input_dir: str, regex: str, recursive: bool):
-    results = []
-    for subitem in os.listdir(input_dir):
-        subitem_path = os.path.join(input_dir, subitem)
-        match = re.match(regex, subitem)
-        if os.path.isdir(subitem_path) and recursive:
-            logging.debug(f'Checking directory "{subitem}"...')
-            results.extend(find_all_candidates(subitem_path, regex, recursive))
-        else:
-            if not match:
-                logging.debug(f'Ignoring "{subitem}"...')
-                continue
-            results.append((subitem_path, subitem, match))
-            logging.debug(f'Collecting "{subitem}"...')
-    return results
-
-
-def rename(
-    csv_path: str,
-    candidates: Iterable[tuple[str, str, re.Match]],
-    current: str,
-    become: str,
-    dry: bool,
-    extension: str,
-    keep_extension: bool,
-):
-    replacement_dict = {}
-    with open(csv_path, "r") as csv_fd:
-        reader = csv.reader(csv_fd)
-        current_col_ind = None
-        become_col_ind = None
-        for row in reader:
-            if current_col_ind is None and become_col_ind is None:
-                current_col_ind = row.index(current)
-                become_col_ind = row.index(become)
-                continue
-            if (
-                row[current_col_ind] in replacement_dict
-                and replacement_dict[row[current_col_ind]] != row[become_col_ind]
-            ):
-                raise Exception("Duplicate current key.")
-            replacement_dict[row[current_col_ind]] = row[become_col_ind]
-    for subitem_path, subitem, match in candidates:
-        if match.group(1) not in replacement_dict:
-            logging.warning(
-                'Group "%s" was not matched to any row in the provided CSV. '
-                "Skipping...",
-                match.group(1),
-            )
-            continue
-        original = subitem_path
-        objective = os.path.join(
-            os.path.dirname(subitem_path),
-            re.sub(match.re, replacement_dict[match.group(1)], subitem.strip()),
-        )
-        if extension:
-            objective += ("." if not extension.startswith(".") else "") + extension
-        if keep_extension:
-            objective += os.path.splitext(subitem_path)[1]
-        logging.info(f'Will rename "{original}" to "{os.path.basename(objective)}"')
-        if os.path.exists(objective):
-            logging.error(
-                f'Path at "{objective}" exists, not continuing. '
-                "Use -f to overwrite instead of stopping."
-            )
-            exit(1)
-        if not dry:
-            os.rename(original, objective)
-    if dry:
-        logging.info("No file names were modified.")
+from renamebycsv.renamer import find_all_candidates, rename_by_csv
 
 
 def run(args):
     candidates = find_all_candidates(args.input_dir, args.regex, args.recursive)
-    rename(
+    rename_by_csv(
         args.csv,
         candidates,
         args.current,
diff --git a/renamebycsv/renamer.py b/renamebycsv/renamer.py
new file mode 100644
index 0000000..ffd6d2c
--- /dev/null
+++ b/renamebycsv/renamer.py
@@ -0,0 +1,77 @@
+import csv
+import logging
+import os
+import re
+from typing import Iterable
+
+
+def find_all_candidates(input_dir: str, regex: str, recursive: bool):
+    results = []
+    for subitem in os.listdir(input_dir):
+        subitem_path = os.path.join(input_dir, subitem)
+        match = re.match(regex, subitem)
+        if os.path.isdir(subitem_path) and recursive:
+            logging.debug(f'Checking directory "{subitem}"...')
+            results.extend(find_all_candidates(subitem_path, regex, recursive))
+        else:
+            if not match:
+                logging.debug(f'Ignoring "{subitem}"...')
+                continue
+            results.append((subitem_path, subitem, match))
+            logging.debug(f'Collecting "{subitem}"...')
+    return results
+
+
+def rename_by_csv(
+    csv_path: str,
+    candidates: Iterable[tuple[str, str, re.Match]],
+    current: str,
+    become: str,
+    dry: bool,
+    extension: str,
+    keep_extension: bool,
+):
+    replacement_dict = {}
+    with open(csv_path, "r") as csv_fd:
+        reader = csv.reader(csv_fd)
+        current_col_ind = None
+        become_col_ind = None
+        for row in reader:
+            if current_col_ind is None and become_col_ind is None:
+                current_col_ind = row.index(current)
+                become_col_ind = row.index(become)
+                continue
+            if (
+                row[current_col_ind] in replacement_dict
+                and replacement_dict[row[current_col_ind]] != row[become_col_ind]
+            ):
+                raise Exception("Duplicate current key.")
+            replacement_dict[row[current_col_ind]] = row[become_col_ind]
+    for subitem_path, subitem, match in candidates:
+        if match.group(1) not in replacement_dict:
+            logging.warning(
+                'Group "%s" was not matched to any row in the provided CSV. '
+                "Skipping...",
+                match.group(1),
+            )
+            continue
+        original = subitem_path
+        objective = os.path.join(
+            os.path.dirname(subitem_path),
+            re.sub(match.re, replacement_dict[match.group(1)], subitem.strip()),
+        )
+        if extension:
+            objective += ("." if not extension.startswith(".") else "") + extension
+        if keep_extension:
+            objective += os.path.splitext(subitem_path)[1]
+        logging.info(f'Will rename "{original}" to "{os.path.basename(objective)}"')
+        if os.path.exists(objective):
+            logging.error(
+                f'Path at "{objective}" exists, not continuing. '
+                "Use -f to overwrite instead of stopping."
+            )
+            exit(1)
+        if not dry:
+            os.rename(original, objective)
+    if dry:
+        logging.info("No file names were modified.")