How do I test output containing dynamic values like dates, GUIDs, or timestamps?

← Testing · Ref: Q208

EK9 expected output files support type-aware placeholders that match dynamic values. Use {{Type}} syntax for values that change between test runs.

PLACEHOLDER SYNTAX

In expected_output.txt, use {{Type}} placeholders:

  Created at: {{DateTime}}
  ID: {{GUID}}
  Count: {{Integer}}

AVAILABLE PLACEHOLDERS

  {{String}}          matches any non-empty text
  {{Integer}}         matches whole numbers
  {{Float}}           matches decimal numbers
  {{Boolean}}         matches true/false
  {{Character}}       matches a single character
  {{Bits}}            matches a run of 0s and 1s
  {{Date}}            matches ISO dates (2026-09-29)
  {{Time}}            matches times (14:30:00)
  {{DateTime}}        matches ISO datetimes with offset
  {{Duration}}        matches ISO durations (P1DT2H)
  {{Millisecond}}     matches millisecond values (250ms)
  {{Money}}           matches amounts with currency (10.50#GBP)
  {{Colour}}          matches hex colours (#FF0000)
  {{Resolution}}      matches dpi/dpc values
  {{Dimension}}       matches dimension values
  {{RegEx}}           matches any non-empty text
  {{GUID}}            matches UUID format
  {{FileSystemPath}}  matches file paths

EXAMPLE EXPECTED OUTPUT FILE

For a program that prints a GUID and timestamp:

  Record ID: {{GUID}}
  Created: {{DateTime}}
  Items: {{Integer}}
  Active: {{Boolean}}

MIXING FIXED AND DYNAMIC

Placeholders can appear anywhere in a line, for example "User alice created at {{DateTime}} with ID {{GUID}}".
Fixed text is matched exactly; each placeholder matches its type's pattern.

See Q204 for black-box testing. See Q205 for parameterized tests. See Q157 for running tests.

Example

defines module qa.testdeep.placeholders

  defines program

    DynamicOutputDemo()
      stdout <- Stdout()

      // === DYNAMIC VALUES IN OUTPUT ===
      // When testing, some values change each run.
      // EK9 expected_output.txt supports placeholders.

      stdout.println("Dynamic output placeholders:")
      stdout.println("  {{String}}   - any non-empty text")
      stdout.println("  {{Integer}}  - whole numbers")
      stdout.println("  {{Float}}    - decimal numbers")
      stdout.println("  {{Boolean}}  - true or false")
      stdout.println("  {{Date}}     - ISO date format")
      stdout.println("  {{DateTime}} - ISO datetime format")
      stdout.println("  {{GUID}}     - UUID format")
      stdout.println("  {{Money}}    - currency format")

      // Example: a test printing dynamic values
      // expected_output.txt would contain:
      //   Record ID: {{GUID}}
      //   Created: {{DateTime}}
      //   Count: {{Integer}}

      stdout.println("Placeholders match dynamic values in output")

Common mistakes

E50010 — Variables must be declared before use. Using stdout before declaring it is a forward reference. See ek9 -h E50010 for details.

Incorrect:

stdout.println("Dynamic output placeholders:")
      stdout <- Stdout()

Correct:

stdout <- Stdout()

      // === DYNAMIC VALUES IN OUTPUT ===

Other ways to ask this

How do I handle non-deterministic output in EK9 tests?
What placeholders can I use in expected output files?
How do I match dynamic values in EK9 test output?
How do I test output containing dynamic values like dates or GUIDs in EK9?
What are output placeholders in EK9 tests?
How do I handle non-deterministic output in EK9 black-box tests?
How do I use {{Date}} and {{GUID}} in expected output files?

Coming from another language?

Java: Hamcrest matchers or regex in assertions. Python: unittest with regex or mock. Rust: custom test helpers with regex. Go: regex assertions in test functions. Kotlin: custom matchers. EK9: type-aware placeholders in expected output files, validated at test runtime.

Keywords: capture, date, anonymous, format, expected, placeholder, black-box, template, output, timestamp, non-deterministic, match, closure, dynamic, guid, coverage, test