Computed Truth
Regex is often called a "write-only" language. A missing escape character or greedy quantifier can cause **Catastrophic Backtracking**, where the engine tries exponential combinations, taking down your server. Understanding the *intent* of a pattern is just as important as writing one that works.
Regex to English Explainer
Decode Pattern
The Technical Proof
Regular expressions are processed by a **Nondeterministic Finite Automaton (NFA)** or DFA. The engine consumes the input string one character at a time, moving through states defined by the pattern.
Common performance killers:
- Greedy Quantifiers (`.*`): Eat everything to the end of the line, then backtrack one char at a time.
- Nested Quantifiers (`(a+)+`): Exponential complexity (O(2^n)).
How to Read Regex
- Anchors First: Look for `^` (Start) and `$` (End). They define the boundaries.
- Groupings: Break down `(...)` sections. These are sub-expressions.
- Quantifiers: `+` (1 or more), `*` (0 or more), `?` (0 or 1). They apply to the *immediately preceding* token.
- Classes: `[...]` defines a set of allowed characters. `\d`, `\w` are shorthand aliases.