Use Regex Pattern Matching in YAML Schema

Pattern Matching in YAML Schema

The pattern keyword validates string values against a regular expression. When converting JSON to YAML Schema, an intelligent converter detects common formats and automatically applies appropriate patterns.

Example JSON Input

{
  "serviceName": "my-api-service",
  "version": "2.1.0",
  "endpoint": "https://api.example.com/v2",
  "apiKey": "sk_live_abc123def456",
  "cronSchedule": "0 */6 * * *"
}

Generated YAML Schema with Patterns

type: object
properties:
  serviceName:
    type: string
    pattern: "^[a-z][a-z0-9-]*[a-z0-9]$"
    description: Lowercase alphanumeric with hyphens
  version:
    type: string
    pattern: "^\\d+\\.\\d+\\.\\d+$"
    description: Semantic version (major.minor.patch)
  endpoint:
    type: string
    format: uri
    pattern: "^https://"
    description: Must be an HTTPS URL
  apiKey:
    type: string
    pattern: "^sk_(live|test)_[a-zA-Z0-9]+$"
    description: API key with environment prefix
  cronSchedule:
    type: string
    pattern: "^[\\d*,/-]+ [\\d*,/-]+ [\\d*,/-]+ [\\d*,/-]+ [\\d*,/-]+$"
    description: Standard 5-field cron expression

Common Patterns Reference

Use case	Pattern
Semver	^\\d+\\.\\d+\\.\\d+$
Slug / kebab-case	^[a-z0-9]+(-[a-z0-9]+)*$
UUID v4	^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$
IPv4 address	^\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}$
ISO date	^\\d{4}-\\d{2}-\\d{2}$
Docker image tag	^[a-z0-9][a-z0-9._/-]*:[a-zA-Z0-9._-]+$

patternProperties for Dynamic Keys

When JSON has dynamic keys that follow a naming pattern:

{
  "env_DATABASE_URL": "postgres://...",
  "env_API_KEY": "sk_live_...",
  "env_LOG_LEVEL": "info"
}

type: object
patternProperties:
  "^env_[A-Z_]+$":
    type: string
additionalProperties: false

This validates that all keys matching the pattern have string values, and no other keys are allowed.

Combining Pattern with Format

endpoint:
  type: string
  format: uri
  pattern: "^https://"

format: uri validates general URI syntax while pattern adds the HTTPS requirement. The two constraints are applied independently (both must pass).

Pattern Limitations

• YAML Schema uses ECMA-262 regex (JavaScript-compatible)
• No lookbehind support in older validators
• Patterns are unanchored by default -- use ^ and $ for full-string matching
• Complex patterns reduce schema readability -- consider enum for small value sets