s = "  Hello, World!  "

print(s.strip())          # 'Hello, World!'
print(s.lower())          # '  hello, world!  '
print(s.upper())
print(s.replace("World", "Python"))
print(s.split(", "))      # ['  Hello', 'World!  ']
print(",".join(["a","b","c"])) # 'a,b,c'

Testing methods

"hello".startswith("he")   # True
"hello123".isalnum()        # True
"   ".isspace()             # True
"Hello".istitle()           # True

# Check and membership
"ell" in "hello"            # True
"hello".find("ll")          # 2  (-1 if not found)
"hello".count("l")          # 2

Strings compare lexicographically using Unicode code points.

print("apple" < "banana")   # True
print("B" < "a")            # True  (uppercase < lowercase)
print(ord("A"), ord("a"))   # 65 97

Sorting strings

words = ["banana", "apple", "Cherry", "date"]

# Default: case-sensitive (uppercase first)
print(sorted(words))
# ['Cherry', 'apple', 'banana', 'date']

# Case-insensitive
print(sorted(words, key=str.lower))
# ['apple', 'banana', 'Cherry', 'date']

# By length
print(sorted(words, key=len))

Function	Returns
re.search(pat, s)	First match object or None
re.findall(pat, s)	List of all matches
re.sub(pat, rep, s)	String with replacements
re.compile(pat)	Compiled pattern object

import re

text = "Call 555-1234 or 555-5678 for info."

# Find all phone numbers
phones = re.findall(r"\d{3}-\d{4}", text)
print(phones)   # ['555-1234', '555-5678']

# Replace
clean = re.sub(r"\d{3}-\d{4}", "XXX-XXXX", text)

Manual approach

word_counter = {}
for word in text.split():
    word = word.strip(".,!?").lower()
    word_counter[word] = word_counter.get(word, 0) + 1

# Sort by frequency
items = sorted(word_counter.items(),
               key=lambda t: t[1], reverse=True)

With Counter

from collections import Counter
import re

words = re.findall(r"\b[a-z]+\b", text.lower())
counts = Counter(words)

# Top 5 words
for word, freq in counts.most_common(5):
    print(freq, word, sep="\t")