Substring Search (find, membership, overlaps)

Substring search answers where (or whether) a pattern appears—in, find, count, or specialized algorithms for many queries. Choose the simplest correct tool until profiling says otherwise.

Why this shows up in the real world

IDE find locates literals; security scanners search signatures inside files.

Core idea (explained for students)

Boolean membership: pat in text. First index: text.find(pat). All starts: loop with start index or regex finditer for pattern objects.

Try this in Python

def contains_any(hay: str, needles: tuple[str, ...]) -> bool:
    return any(n in hay for n in needles)


print(contains_any("server-01.log", (".err", ".fail")))

Common mistakes

• Overlapping occurrences—find stepping by +1 vs +len(pat) changes results.
• Empty pat edge cases returning 0 repeatedly in some APIs.

Key takeaways

• Start naive, then graduate to KMP/Rabin–Karp when lengths and query counts grow.
• Preprocess with suffix array only for advanced competitive constraints.