azp: Add Substring

src/Sdk/AzurePipelines/descriptions.json

@@ -115,6 +115,9 @@
     },
     "trim": {
       "description": "* Returns the parameter without leading and trailing white spaces\n* Min parameters: 1. Max parameters: 1\n* Example: `trim('  variable  ') ` returns 'variable'"
+    },
+    "substring": {
+      "description": "* Returns a substring from the input string, starting at the specified index and having the specified length.\n* Min parameters: 3. Max parameters: 3\n* The first parameter is the input string. The second parameter is the zero-based starting index of the substring. The third parameter is the length of the substring.\n* Example: `substring('Hello world', 0, 5)` returns 'Hello'"
     }
   }
 }

src/Sdk/DTExpressions2/Expressions2/ExpressionConstants.cs

@@ -45,6 +45,8 @@ static ExpressionConstants()
             // https://learn.microsoft.com/en-us/azure/devops/release-notes/2025/pipelines/sprint-248-update#new-pipeline-expression-functions
             AddAzureFunction<GitHub.DistributedTask.Expressions2.Sdk.Functions.v1.Iif>("iif", 3, 3);
             AddAzureFunction<GitHub.DistributedTask.Expressions2.Sdk.Functions.v1.Trim>("trim", 1, 1);
+            // Undocumented: 
+            AddAzureFunction<GitHub.DistributedTask.Expressions2.Sdk.Functions.v1.SubString>("substring", 3, 3);
         }
 
         private static void AddFunction<T>(String name, Int32 minParameters, Int32 maxParameters)

src/Sdk/DTExpressions2/Expressions2/Sdk/Functions/v1/SubString.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace GitHub.DistributedTask.Expressions2.Sdk.Functions.v1
+{
+    internal sealed class SubString : Function
+    {
+        protected sealed override Boolean TraceFullyRealized => false;
+
+        protected sealed override Object EvaluateCore(EvaluationContext context, out ResultMemory memory)
+        {
+            memory = null;
+            String left = Parameters[0].EvaluateString(context) as String ?? String.Empty;
+            Int32 startIndex = (Int32)Parameters[1].Evaluate(context).ConvertToNumber();
+            Int32 length = (Int32)Parameters[2].Evaluate(context).ConvertToNumber();
+            return left.Substring(startIndex, length);
+        }
+    }
+}

testworkflows/azpipelines/substring-function-1/pipeline.yml

@@ -0,0 +1,11 @@
+trigger: none
+pr: none
+
+pool:
+  vmImage: 'ubuntu-latest'
+
+steps:
+- ${{ if ne(substring('Hello small World', 6, 5), 'small') }}:
+  - fail: true
+- ${{ else }}:
+  - bash: echo "substring function works as expected"

testworkflows/azpipelines/substring-function/pipeline.yml

@@ -0,0 +1,46 @@
+trigger: none
+pr: none
+
+variables:
+  Tag: $[ substring(variables['Build.SourceVersion'], 0, 7) ]
+  ShortHash: $[ substring(variables['Build.SourceVersion'], 0, 7) ]
+  StaticSubstring: $[ substring('abcdef1234567890', 0, 7) ]
+
+pool:
+  vmImage: 'ubuntu-latest'
+
+steps:
+- checkout: none
+
+- bash: |
+    set -euo pipefail
+
+    echo "Build.SourceVersion: ${SOURCE_VERSION}"
+    echo "Tag: ${TAG_VALUE}"
+    echo "ShortHash: ${SHORT_HASH_VALUE}"
+    echo "StaticSubstring: ${STATIC_SUBSTRING_VALUE}"
+
+    expected_short_hash="${SOURCE_VERSION:0:7}"
+
+    if [ "${TAG_VALUE}" != "${expected_short_hash}" ]; then
+      echo "##vso[task.logissue type=error]Tag did not match expected short hash '${expected_short_hash}'"
+      exit 1
+    fi
+
+    if [ "${SHORT_HASH_VALUE}" != "${expected_short_hash}" ]; then
+      echo "##vso[task.logissue type=error]ShortHash did not match expected short hash '${expected_short_hash}'"
+      exit 1
+    fi
+
+    if [ "${STATIC_SUBSTRING_VALUE}" != "abcdef1" ]; then
+      echo "##vso[task.logissue type=error]StaticSubstring did not match expected value 'abcdef1'"
+      exit 1
+    fi
+
+    echo "substring runtime expressions evaluated successfully."
+  displayName: 'Test: substring runtime expression'
+  env:
+    SOURCE_VERSION: $(Build.SourceVersion)
+    TAG_VALUE: $(Tag)
+    SHORT_HASH_VALUE: $(ShortHash)
+    STATIC_SUBSTRING_VALUE: $(StaticSubstring)