str.replace() / str.translate()

# Replaces all occurrences of old with new in the string (returns a new string).
str.replace(old, new, count=-1)

# Creates a translation table for multi-character substitution.
str.maketrans(x, y=None, z=None)

# Replaces characters using a translation table created with maketrans().
str.translate(table)

# Replace a substring using replace().
text = "Python is hard. Let's learn Python. Python is wonderful."
result = text.replace("hard", "fun")
print(result)  # Python is fun. Let's learn Python. Python is wonderful.

# Limit the number of replacements with count.
text2 = "a b c a b c a"
print(text2.replace("a", "X", 2))  # X b c X b c a (only the first 2 are replaced)

# To delete characters, replace them with an empty string.
html_like = "Hello, <b>Alice</b>!"
clean = html_like.replace("<b>", "").replace("</b>", "")
print(clean)  # Hello, Alice!

# replace() does not modify the original string (strings are immutable).
original = "hello"
new = original.replace("l", "r")
print(original)  # hello (unchanged)
print(new)       # herro

# Use maketrans() and translate() to convert multiple characters at once.
# A simple example converting lowercase vowels to uppercase.
table = str.maketrans("aeiou", "AEIOU")
print("hello world".translate(table))  # hEllO wOrld

# Convert digits to their fullwidth equivalents.
digit_table = str.maketrans("0123456789", "０１２３４５６７８９")
print("2026-03-05".translate(digit_table))  # ２０２６－０３－０５

# Delete specific characters (characters in the third argument are removed).
remove_table = str.maketrans("", "", "aeiou")  # Remove vowels.
print("hello world".translate(remove_table))  # hll wrld

# You can also specify a translation table using a dictionary (with Unicode code points).
table2 = str.maketrans({"A": "1", "B": "2", "C": "3"})
print("ABCABC".translate(table2))  # 123123

# A practical example: normalize mixed line endings.
mixed_newlines = "line1\r\nline2\rline3\nline4"
unified = mixed_newlines.replace("\r\n", "\n").replace("\r", "\n")
print(repr(unified))  # Normalized to '\n' line endings.

Python strings are immutable (cannot be changed in place). replace() does not modify the original string — it returns a new string object with the replacements applied. To update the original variable, reassign it: text = text.replace(old, new).

When replacing multiple substrings in sequence, chaining replace() calls (method chaining) is the common approach. However, when the number of replacements is large, translate() is faster. For text cleansing — bulk deletion or conversion of unwanted characters — translate() is particularly effective.

For advanced pattern matching and replacement using regular expressions, use re.sub() from the standard library's re module. For example, to collapse consecutive whitespace into a single space, use re.sub(r"\s+", " ", text). For string searching, see str.find() / str.index().