Added swap case program and removed unexpected expression part

strings/swap_case.py

@@ -0,0 +1,42 @@
+"""
+This algorithm helps you to swap cases.
+
+User will give input and then program will perform swap cases.
+
+In other words, convert all lowercase letters to uppercase letters and vice versa.
+For example:
+1. Please input sentence: Algorithm.Python@89
+  aLGORITHM.pYTHON@89
+2. Please input sentence: github.com/mayur200
+  GITHUB.COM/MAYUR200
+
+"""
+import re
+
+# This re.compile() function saves the pattern from 'a' to 'z' and 'A' to 'Z'
+# into 'regexp' variable
+regexp = re.compile("[^a-zA-Z]+")
+
+
+def swap_case(sentence):
+    """
+    This function will convert all lowercase letters to uppercase letters
+    and vice versa.
+
+    >>> swap_case('Algorithm.Python@89')
+    'aLGORITHM.pYTHON@89'
+    """
+    new_string = ""
+    for char in sentence:
+        if char.isupper():
+            new_string += char.lower()
+        if char.islower():
+            new_string += char.upper()
+        if regexp.search(char):
+            new_string += char
+
+    return new_string
+
+
+if __name__ == "__main__":
+    print(swap_case(input("Please input sentence:")))