MEPX
Chapter 1 of 10All chapters
  1. 1What a regex is
  2. 2Literals and classes
  3. 3Quantifiers
  4. 4Anchors and boundaries
  5. 5Groups and alternation
  6. 6Escaping
  7. 7Lookaround
  8. 8Flags
  9. 9When regex goes wrong
  10. 10Writing readable patterns

Chapter 1 of 10

What a regex is

Patterns, not programs.

The idea

A regular expression describes a set of strings. The engine walks the text trying to match that description, and reports where it succeeded.

  • The same pattern language appears in editors, grep, and almost every programming language.
  • Flavours differ in the corners, so a pattern is not always portable.

Where it fits

Regex is right for flat, predictable text: log lines, identifiers, simple formats. It cannot describe nesting, which is why HTML and JSON need real parsers.