Case-insensitive replacement
Input
replace_ignore_case("FooBarfoo", "foo", "x")
Output
xBarx
Both 'Foo' and 'foo' are replaced with 'x'.
Full lesson preview
Replace a substring ignoring case
Replace all occurrences of a substring regardless of case, preserving non-matching text.
Python practice15 minString PatternsIntermediateLast updated March 18, 2026
Problem statement
Write a function replace_ignore_case(s, old, new) that returns a new string where every occurrence of old in s is replaced with new, ignoring case when matching old. Replacements should be non-overlapping (standard left-to-right behavior). If old is an empty string, return s unchanged.
Use only the Python standard library.
Examples:
- replace_ignore_case('Hello World', 'hello', 'hi') -> 'hi World'
- replace_ignore_case('Spam spam SPAM', 'spam', 'eggs') -> 'eggs eggs eggs'
Task
Implement replace_ignore_case(s, old, new) to replace all case-insensitive occurrences of old in s with new (non-overlapping).
Examples
Input format
Three strings: s, old, new passed as replace_ignore_case(s, old, new).
Output format
Return a single string with all case-insensitive replacements applied (non-overlapping).
Constraints
- 0 <= len(s), len(old), len(new) <= 5000
- If old is empty, return s unchanged
- Matching is case-insensitive, replacement uses new verbatim
Samples
Sample 1
Input
replace_ignore_case("Spam spam SPAM", "spam", "eggs")
Output
eggs eggs eggs
All three case variants of 'spam' are replaced by 'eggs'.