string-matching

An algorithm guide for finding exact occurrences of a pattern in text, including methods that reuse earlier comparisons or build indexes for repeated searches.

In plain words
What is it for?
Use it for KMP, rolling-hash, finite-automaton, suffix-array, longest-substring, cyclic-rotation, or Burrows–Wheeler transform problems.
Why use it?
It helps choose a search method that fits the text, pattern, hashing assumptions, and number of queries instead of comparing characters unnecessarily.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/arcadi4/nerdy/string-matching
Any agent
npx skills add Arcadi4/nerdy --skill string-matching
Clone the repo
git clone --depth 1 https://github.com/Arcadi4/nerdy

Made for: Claude Code, Codex.

Per session 56 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,868 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00056 $0.03868
Opus 5 $0.00028 $0.01934
Sonnet 5 $0.00011 $0.00774
Haiku 4.5 $0.00006 $0.00387

Measured 2d ago against content hash 58bb8b7be660, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

string-matching scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

clrs/string-matching/SKILL.md · 365 lines

How it starts

The opening of the file, as written. The whole thing — 365 lines — stays where its author put it; the contents beside it link to each section on GitHub.

String Matching

Overview

String matching is about preserving information from previous character comparisons. The right method depends on whether the text is scanned once, patterns repeat, hashes are acceptable as filters, a full automaton is affordable, or the corpus should be indexed for many later queries.

Core principle: choose the matcher by workload and model assumptions, then state the invariant that makes skipped comparisons safe.

Shared CLRS Conventions

Follow the parent clrs skill for mathematical formatting, formula-free headings, direct polished answers, and CLRS-wide answer style. Keep string indices, costs, recurrences, and asymptotic bounds in display LaTeX blocks rather than inline prose.

When a table needs a cost or formula, keep the cell verbal and place the expression in a display block nearby.

For pressure-test answers, treat concrete assignments and indexed values as formulas too. Do not write prefix-function updates, array entries, or length equalities inline; introduce them in prose and put the symbolic expression in a display block.

When to Use

  • A prompt asks for exact pattern occurrences, valid shifts, overlaps, string-matching automata, Rabin-Karp, KMP, suffix arrays, LCP arrays, cyclic rotations, or Burrows-Wheeler transforms.
  • You need to compare naive matching, rolling hashes, finite automata, KMP, and suffix-array indexing under a workload constraint.
  • A proof asks why a shift is safe after a mismatch, why automaton states mean longest matched prefixes, or why suffix-array matches form a consecutive block.
  • A production question involves collision risk, adversarial text, large alphabets, static corpora, many pattern queries, or reuse of learned comparisons.

Do not use this skill for approximate matching, edit distance, regular-expression engines, suffix trees, tries, or general dynamic programming unless exact substring search is the core issue.

String-Matching Model

Name the text, pattern, alphabet, and valid shift before choosing an algorithm.

Read the full file on GitHub · 365 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 365 lines · 56 tokens per session scan A 58bb8b7be660

Subscribe to this mod's changes

string-matching is a skill published in the GitHub repository Arcadi4/nerdy (7 stars, last pushed 4mo ago), licensed MIT. It adds 56 tokens to every session and 3,868 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.