Regular Expressions and Finite Automata

**Regular Expressions (Regex)** and **Finite Automata** are foundational concepts in computer science used for pattern matching and text processing. In Wikantik, these tools are used extensively for parsing Markdown, scanning links, and validating metadata.

Finite Automata

A Finite Automaton is a mathematical model of computation. It consists of a finite number of states and transitions between them based on input symbols.

- **Deterministic (DFA):** For every state and input, there is exactly one transition.

- **Non-deterministic (NFA):** Can have multiple transitions for the same input or "epsilon" transitions (without input).

Regular Expressions

Regex is a formal language used to describe sets of strings. Every regular expression can be converted into an equivalent Finite Automaton (and vice versa).

Common Syntax

- `.` (Any character)

- `*` (Zero or more)

- `+` (One or more)

- `[a-z]` (Character class)

- `^` / `$` (Anchors)

Applications in Wikantik

1. Markdown Parsing

The **Flexmark** parser used by Wikantik uses complex regular expressions to identify headings, bold text, links, and code blocks within the Markdown source.

2. Link Scanning

The `MarkdownLinkScanner` (in `wikantik-api`) uses regex to find internal wiki links (e.g., `[PageName](PageName)`) and external URLs.

- **Example:** `\[\[([^|\]]+)(?:\|([^\]]+))?\]\]` matches standard wiki brackets with optional display text.

3. Frontmatter Validation

Wikantik uses regex to validate the format of mandatory fields like `canonical_id` (ensuring it is a 26-character ULID) and `date`.

4. Search and Retrieval

While the primary search is BM25-based, regex can be used in administrative tools to perform "power searches" across the corpus for specific patterns or legacy JSPWiki constructs.