MEPX
Chapter 5 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 5 of 10

Groups and alternation

Structure inside a pattern.

Capturing

Parentheses group parts together and capture what they matched, numbered from left to right. Captures are how you extract fields rather than just testing for a match.

  • Non-capturing groups start with ?: and keep numbering meaningful.
  • Named groups make replacement strings readable months later.

Alternation

A vertical bar means or, and it applies to the whole pattern unless grouped. Wrapping alternatives in a group is nearly always what you meant.