Challenge: Write a Template Parser

This is my solution to a template parsing challenge. I implemented the core of the parsing/interpolation logic using regular expressions with capture groups and back references. The main parse method then iteratively substitutes environment values and captured matches until there are no captures.

The parser supports plain variables, if and unless type logic, as well as nested captures. The parser may be instantiated once with an environment and it can parse multiple templates with that environment. It will always operate on and return a copy of the template.

Description

Given a template and an environment, fill the template with correct values.

# template is a string.
# environment is a dictionary that specifies the variables.
def parse(template, environment)
  # ...code...
end

Running Specs

1. Clone the repo:

git clone git@github.com:DiegoSalazar/template_parsing_challenge.git
cd template_parsing_challenge

2. Run bundle unless you already have rspec installed.
3. Run rspec ./template_parser_spec.rb

Examples

# Input:
template: "Hi {FIRST_NAME}"
environment: { first_name: "Diego" }

# Output:
"Hi Diego"

# Input:
template: "Hi {FIRST_NAME}"
environment: { }

# Output:
"Hi"

# Input:
template: "{#PATIENT_PAID}Patient paid.{#PATIENT_PAID}"
environment: { patient_paid: true }

# Output:
"Patient paid."

# Input:
template: "{#PATIENT_PAID}Patient paid.{#PATIENT_PAID}"
environment: { patient_paid: false }

# Output:
""

# Input:
template: "{^PATIENT_PAID}Patient did not pay.{^PATIENT_PAID}"
environment: { patient_paid: false }

# Output:
"Patient did not pay."

# Input:
template: "{^PATIENT_PAID}Patient did not pay.{^PATIENT_PAID}"
environment: { patient_paid: true }

# Output:
""

# Input:
template: "{#PATIENT_PAID}Patient paid.{^PATIENT_YOUNG} Patient is not young.{^PATIENT_YOUNG}{#PATIENT_PAID}"
environment: { patient_paid: true, patient_young: true }

# Output:
"Patient paid."

# Input:
template: "{#PATIENT_PAID}Patient paid. {^PATIENT_YOUNG} Patient is not young.{^PATIENT_YOUNG}{#PATIENT_PAID}"
environment: { patient_paid: true, patient_young: false }

# Output:
"Patient paid. Patient is not young."

# Input:
template: "{#PATIENT_PAID}Patient paid. {^PATIENT_YOUNG} Patient is not young.{^PATIENT_YOUNG}{#PATIENT_PAID}"
environment: { patient_paid: false, patient_young: false }

# Output:
""