Add substring() function

docs/reference/schemas/config/functions/substring.md

@@ -0,0 +1,267 @@
+---
+description: Reference for the 'substring' DSC configuration document function
+ms.date:     09/27/2025
+ms.topic:    reference
+title:       substring
+---
+
+# substring
+
+## Synopsis
+
+Returns a substring that starts at the specified character position and contains
+the specified number of characters.
+
+## Syntax
+
+```Syntax
+substring(<stringToParse>, <startIndex>)
+substring(<stringToParse>, <startIndex>, <length>)
+```
+
+## Description
+
+The `substring()` function extracts a portion of a string based on the specified
+starting position and optional length. The function uses zero-based indexing,
+meaning the first character is at position 0. This is useful for parsing
+identifiers, extracting prefixes or suffixes, manipulating configuration values,
+or formatting display strings.
+
+Key behaviors:
+
+- **Zero-based indexing**: The first character is at index 0
+- **Optional length**: If length is omitted, returns the remainder of the string
+  from the start position
+- **Boundary validation**: Prevents access beyond string boundaries with clear
+  error messages
+
+## Examples
+
+### Example 1 - Extract environment from resource name
+
+This example demonstrates extracting environment information from standardized
+resource names for conditional configuration. It uses the [`parameters()`][08]
+function to retrieve the resource name.
+
+```yaml
+# substring.example.1.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  resourceName:
+    type: string
+    defaultValue: svc-api-prod-east
+resources:
+- name: Extract environment
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      environment: "[substring(parameters('resourceName'), 8, 4)]"
+```
+
+```bash
+dsc config get --file substring.example.1.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Extract environment
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        environment: prod
+messages: []
+hadErrors: false
+```
+
+### Example 2 - Extract region from resource identifier
+
+This example shows extracting a region code from a standardized resource
+identifier without specifying length, using [`parameters()`][08] to retrieve
+the identifier.
+
+```yaml
+# substring.example.2.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  resourceId:
+    type: string
+    defaultValue: app-web-eastus2-001
+resources:
+- name: Extract region
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      region: "[substring(parameters('resourceId'), 8)]"
+```
+
+```bash
+dsc config get --file substring.example.2.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Extract region
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        region: eastus2-001
+messages: []
+hadErrors: false
+```
+
+### Example 3 - Parse version components
+
+This example demonstrates parsing semantic version strings to extract major,
+minor, and patch components. It uses [`parameters()`][08] to retrieve the
+version string.
+
+```yaml
+# substring.example.3.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  version:
+    type: string
+    defaultValue: "3.2.1"
+resources:
+- name: Parse version
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      major: "[substring(parameters('version'), 0, 1)]"
+      minor: "[substring(parameters('version'), 2, 1)]"
+      patch: "[substring(parameters('version'), 4, 1)]"
+```
+
+```bash
+dsc config get --file substring.example.3.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Parse version
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        major: "3"
+        minor: "2"
+        patch: "1"
+messages: []
+hadErrors: false
+```
+
+### Example 4 - Unicode and emoji support
+
+This example shows that `substring()` correctly handles Unicode characters and
+emojis. It uses [`parameters()`][08] to retrieve the message string.
+
+```yaml
+# substring.example.4.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  message:
+    type: string
+    defaultValue: "Hello 🌍 World!"
+resources:
+- name: Unicode substring
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      greeting: "[substring(parameters('message'), 0, 5)]"
+      emoji: "[substring(parameters('message'), 6, 1)]"
+      remainder: "[substring(parameters('message'), 8)]"
+```
+
+```bash
+dsc config get --file substring.example.4.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Unicode substring
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        greeting: Hello
+        emoji: 🌍
+        remainder: " World!"
+messages: []
+hadErrors: false
+```
+
+## Parameters
+
+### stringToParse
+
+The original string from which the substring is extracted.
+
+```yaml
+Type:     string
+Required: true
+Position: 1
+```
+
+### startIndex
+
+The zero-based starting character position for the substring. Must be a
+non-negative integer and cannot exceed the length of the string.
+
+```yaml
+Type:     int
+Required: true
+Position: 2
+```
+
+### length
+
+The number of characters for the substring. Must be a non-negative integer. The
+start index plus length cannot exceed the length of the string. If omitted, the
+remainder of the string from the start position is returned.
+
+```yaml
+Type:     int
+Required: false
+Position: 3
+```
+
+## Output
+
+The `substring()` function returns a string containing the extracted portion of
+the original string.
+
+```yaml
+Type: string
+```
+
+## Exceptions
+
+The `substring()` function raises errors for the following conditions:
+
+- **Invalid start index**: When `startIndex` is negative
+- **Start index out of bounds**: When `startIndex` exceeds the string length
+- **Invalid length**: When `length` is negative
+- **Length out of bounds**: When `startIndex + length` exceeds the string
+  length
+
+## Related functions
+
+- [`string()`][00] - Converts values to strings
+- [`concat()`][01] - Concatenates strings together
+- [`indexOf()`][02] - Finds the index of a substring in a string
+- [`lastIndexOf()`][03] - Finds the last index of a substring in a string
+- [`length()`][04] - Returns the length of a string or array
+- [`startsWith()`][05] - Checks if a string starts with a prefix
+- [`endsWith()`][06] - Checks if a string ends with a suffix
+
+<!-- Link reference definitions -->
+[00]: ./string.md
+[01]: ./concat.md
+[02]: ./indexOf.md
+[03]: ./lastIndexOf.md
+[04]: ./length.md
+[05]: ./startsWith.md
+[06]: ./endsWith.md
+[08]: ./parameters.md

dsc/tests/dsc_functions.tests.ps1

@@ -660,6 +660,56 @@ Describe 'tests for function expressions' {
               type: Microsoft.DSC.Debug/Echo
               properties:
                 output: "$expression"
+"@
+    $out = dsc -l trace config get -i $config_yaml 2>$TestDrive/error.log
+    $LASTEXITCODE | Should -Not -Be 0
+    $errorContent = Get-Content $TestDrive/error.log -Raw
+    $errorContent | Should -Match ([regex]::Escape($expectedError))
+  }
+
+  It 'substring function works for: <expression>' -TestCases @(
+    @{ expression = "[substring('hello world', 6, 5)]"; expected = 'world' }
+    @{ expression = "[substring('hello', 0, 2)]"; expected = 'he' }
+    @{ expression = "[substring('hello', 1, 3)]"; expected = 'ell' }
+    @{ expression = "[substring('hello', 2)]"; expected = 'llo' }
+    @{ expression = "[substring('hello', 0)]"; expected = 'hello' }
+    @{ expression = "[substring('hello', 5)]"; expected = '' }
+    @{ expression = "[substring('hello', 1, 1)]"; expected = 'e' }
+    @{ expression = "[substring('hello', 5, 0)]"; expected = '' }
+    @{ expression = "[substring('', 0)]"; expected = '' }
+    @{ expression = "[substring('', 0, 0)]"; expected = '' }
+    @{ expression = "[substring('héllo', 1, 2)]"; expected = 'él' }
+  ) {
+    param($expression, $expected)
+
+    $escapedExpression = $expression -replace "'", "''"
+    $config_yaml = @"
+            `$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+            resources:
+            - name: Echo
+              type: Microsoft.DSC.Debug/Echo
+              properties:
+                output: '$escapedExpression'
+"@
+    $out = $config_yaml | dsc config get -f - | ConvertFrom-Json
+    $out.results[0].result.actualState.output | Should -Be $expected
+  }
+
+  It 'substring function error handling: <expression>' -TestCases @(
+    @{ expression = "[substring('hello', -1, 2)]"; expectedError = 'Start index cannot be negative' }
+    @{ expression = "[substring('hello', 1, -1)]"; expectedError = 'Length cannot be negative' }
+    @{ expression = "[substring('hello', 10, 1)]"; expectedError = 'Start index is beyond the end of the string' }
+    @{ expression = "[substring('hello', 2, 10)]"; expectedError = 'Length extends beyond the end of the string' }
+  ) {
+    param($expression, $expectedError)
+
+    $config_yaml = @"
+            `$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+            resources:
+            - name: Echo
+              type: Microsoft.DSC.Debug/Echo
+              properties:
+                output: `"$expression`"
 "@
     $out = dsc -l trace config get -i $config_yaml 2>$TestDrive/error.log
     $LASTEXITCODE | Should -Not -Be 0

dsc_lib/locales/en-us.toml

@@ -479,6 +479,16 @@ invalidOriginalValue = "First argument must be an array or string"
 [functions.string]
 description = "Converts a value to a string"
 
+[functions.substring]
+description = "Returns a substring that starts at the specified character position and contains the specified number of characters"
+invoked = "substring function"
+startIndexNegative = "Start index cannot be negative"
+startIndexTooLarge = "Start index is beyond the end of the string"
+startIndexValueTooLarge = "Start index value too large"
+lengthNegative = "Length cannot be negative"
+lengthTooLarge = "Length extends beyond the end of the string"
+lengthValueTooLarge = "Length value too large"
+
 [functions.sub]
 description = "Subtracts the second number from the first"
 invoked = "sub function"

dsc_lib/src/functions/mod.rs

@@ -60,6 +60,7 @@ pub mod skip;
 pub mod starts_with;
 pub mod string;
 pub mod sub;
+pub mod substring;
 pub mod system_root;
 pub mod r#true;
 pub mod union;
@@ -177,6 +178,7 @@ impl FunctionDispatcher {
             Box::new(starts_with::StartsWith{}),
             Box::new(string::StringFn{}),
             Box::new(sub::Sub{}),
+            Box::new(substring::Substring{}),
             Box::new(system_root::SystemRoot{}),
             Box::new(r#true::True{}),
             Box::new(utc_now::UtcNow{}),