diff --git a/strings/dna.py b/strings/dna.py
deleted file mode 100644
index 46e271d689db..000000000000
--- a/strings/dna.py
+++ /dev/null
@@ -1,26 +0,0 @@
-import re
-
-
-def dna(dna: str) -> str:
-
-    """
-    https://en.wikipedia.org/wiki/DNA
-    Returns the second side of a DNA strand
-
-    >>> dna("GCTA")
-    'CGAT'
-    >>> dna("ATGC")
-    'TACG'
-    >>> dna("CTGA")
-    'GACT'
-    >>> dna("GFGG")
-    'Invalid Strand'
-    """
-
-    r = len(re.findall("[ATCG]", dna)) != len(dna)
-    val = dna.translate(dna.maketrans("ATCG", "TAGC"))
-    return "Invalid Strand" if r else val
-
-
-if __name__ == "__main__":
-    __import__("doctest").testmod()
diff --git a/strings/dna_matching_strand.py b/strings/dna_matching_strand.py
new file mode 100644
index 000000000000..6de07b202073
--- /dev/null
+++ b/strings/dna_matching_strand.py
@@ -0,0 +1,26 @@
+def dna_matching_strand(dna: str) -> str:
+    """
+    https://en.wikipedia.org/wiki/DNA
+    Returns the second side of a DNA strand
+
+    >>> dna_matching_strand("GCTA")
+    'CGAT'
+    >>> dna_matching_strand("ATGC")
+    'TACG'
+    >>> dna_matching_strand("CTGA")
+    'GACT'
+    >>> dna_matching_strand("GFGG")
+    Traceback (most recent call last):
+        ...
+    ValueError: GFGG is not a valid strand of DNA
+    """
+    dna = dna.upper()
+    if not all(c in "ATGC" for c in dna):
+        raise ValueError(f"{dna} is not a valid strand of DNA")
+    return dna.translate(str.maketrans("ATGC", "TACG"))
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()