feat: Add interpolation functions to Hiera

[isaiahfrantz]

Add String Manipulation and Pattern Matching Interpolation Functions

Summary

This PR adds a suite of new interpolation functions to Hiera, enabling powerful string manipulation, pattern matching, and version comparison directly within data values and hierarchy paths.
This change will solve a major pain point with hera by providing a way to construct data file names that can be more easily matched against without needing unnecessary new facts or global variables.

New Features

1. substring(string, start[, count])

Extract a portion of a string using 0-based indexing.

slice: "%{substring('abcdef', 2)}"      # Returns 'cdef'
slice: "%{substring('abcdef', 2, 3)}"   # Returns 'cde'
last3: "%{substring('abcdef', -3)}"     # Returns 'def' (negative index)

# In hierarchy paths
path: "nodes/%{substring($certname, 0, 4)}.yaml"

2. match(string, pattern)

Test if a string matches a regex or contains a substring. Returns 'true' or 'false'.

is_web: "%{match($role, '/^web/')}"
has_db: "%{match($hostname, 'db')}"
case_insensitive: "%{match('HELLO', '/hello/i')}"

# In hierarchy paths
path: "nodes/%{match($certname, 'db')}.yaml"

Supports regex flags: i (case-insensitive), m (multiline), x (extended)

3. grep_captures(string, '/regex/'[, separator][, [groups]])

Extract capture groups from a regex match and join them.

# All groups concatenated (default)
all: "%{grep_captures('abc-123-def', '/([a-z]+)-(\d+)-([a-z]+)/')}"
# Returns: 'abc123def'

# All groups with separator
sep: "%{grep_captures('abc-123-def', '/([a-z]+)-(\d+)-([a-z]+)/', '-')}"
# Returns: 'abc-123-def'

# Specific groups (3rd arg auto-detected as array)
selected: "%{grep_captures('abc-123-def', '/([a-z]+)-(\d+)-([a-z]+)/', [1,3])}"
# Returns: 'abcdef'

# Specific groups with separator
custom: "%{grep_captures('abc-123-def', '/([a-z]+)-(\d+)-([a-z]+)/', '_', [1,3])}"
# Returns: 'abc_def'

# In hierarchy paths
path: "env/%{grep_captures($hostname, '/\w+-(\w+)-\w+/')}.yaml"

Key features:

• Default separator: '' (empty string)
• Default groups: all capture groups
• Third argument auto-detected: array = groups, quoted string = separator

4. Version Comparison Functions

Compare semantic versions using Gem::Version:

is_newer: "%{version_gt($app_version, '2.0.0')}"
is_compatible: "%{version_gte($ruby_version, '2.7.0')}"
needs_upgrade: "%{version_lt($os_version, '8.0')}"
is_current: "%{version_lte($version, '3.0.0')}"

# In hierarchy paths
path: "versions/%{version_gte($app_version, '2.0.0')}.yaml"

• Handles semantic versioning correctly (1.10.0 > 1.9.0)
• Supports prerelease versions (1.0.0.alpha < 1.0.0)

Technical Changes

Changed

• RX_METHOD_AND_ARG regex updated to support function arguments containing parentheses
• Argument parser now handles array literals [1,2,3] for capture group indices

Fixed

• Regex literals with escaped slashes (e.g., /a\/b/) now parsed correctly
• Empty quoted strings ('' or "") handled correctly as function arguments

Single-Quoted Regex Patterns

All regex patterns should use single quotes ('/pattern/') for:

• Curly brace quantifier support: '/\d{3,5}/'
• No YAML double-escaping needed: '/\d+/' instead of /\\d+/

Files Changed

• lib/hiera/interpolate.rb - Core implementation
• spec/unit/interpolate_spec.rb - Tests
• spec/unit/fixtures/interpolate/data/weird_keys.yaml - Test fixtures
• README.md - Documentation
• CHANGELOG.md - Changelog
• docs/tutorials/interpolation_functions.md - Tutorial

Testing

bundle exec rspec spec/unit/interpolate_spec.rb
# 86 examples, 0 failures

Use Cases

All functions can be used in both data values and hierarchy paths.

Dynamic hierarchy paths using all functions:

# hiera.yaml
:hierarchy:
  # substring - group nodes by certname prefix
  - name: "node prefix"
    path: "nodes/%{substring($certname, 0, 4)}.yaml"

  # grep_captures - extract environment from structured hostname (web01-prod-dc1)
  - name: "environment from hostname"
    path: "env/%{grep_captures($hostname, '/\w+-(\w+)-\w+/')}.yaml"

  # match - conditional path based on pattern (returns 'true' or 'false')
  - name: "production check"
    path: "is_prod/%{match($environment, '/^prod/')}.yaml"

  # version functions - path based on version comparison
  - name: "version compatibility"
    path: "compat/%{version_gte($app_version, '2.0.0')}.yaml"

  - name: "common"
    path: "common.yaml"

Data value examples:

# Hostname: web01-prod-dc1.example.com
env: "%{grep_captures($hostname, '/\w+-(\w+)-\w+/')}"  # Returns 'prod'
dc: "%{grep_captures($hostname, '/\w+-\w+-(\w+)/')}"   # Returns 'dc1'
short_name: "%{substring($certname, 0, 8)}"
is_webserver: "%{match($role, '/^web/')}"
use_new_api: "%{version_gte($app_version, '2.0.0')}"

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[trefzer]

just fyi, I 'solved' the equal pai point you described with an custom hiera backend, that allow to run any puppet function.
It's available at github: https://github.com/cirrax/hiera-pfunc. There are also the slides available from the talk about it I gave on monda at cfgmgmt camp in ghent.