This script can be added as a task in pull request validation build with a branching pattern defined as a variable. For more information see http://chamindac.blogspot.com/2019/05/controlling-pull-request-source.html

PRBranchPatternControlinPRvalidationBuildforAzureGitRepos.ps1

$sourceBranch = $env:System_PullRequest_SourceBranch
$targetBranch = $env:System_PullRequest_TargetBranch

$branchControlPattern = $(BranchControlPattern)



Write-Host ('PR Source Branch: ' + $sourceBranch)
Write-Host ('PR Target Branch: ' + $targetBranch)

$sourceBranchFilter = $sourceBranch -replace 'refs/heads/', ''
$targetBranchFilter = $targetBranch -replace 'refs/heads/', ''

if($sourceBranchFilter.Contains("/"))
{
    $sourceBranchFilter = $sourceBranchFilter -replace $sourceBranchFilter.Substring($sourceBranchFilter.IndexOf("/")),'/*'
}

if($targetBranchFilter.Contains("/"))
{
    $targetBranchFilter = $targetBranchFilter -replace $targetBranchFilter.Substring($targetBranchFilter.IndexOf("/")),'/*'
}

Write-Host ('PR Source Branch Filter: ' + $sourceBranchFilter)
Write-Host ('PR Target Branch Filter: ' + $targetBranchFilter)

$possibleSourceBranchPatterns = $branchControlPattern.Item($targetBranchFilter)

Write-Host ('PR Possible Source Branch Patterns: ' + $possibleSourceBranchPatterns)

if ($possibleSourceBranchPatterns -eq $null)
{
    Write-Error ('PR No possible source branch pattern found for target branch: ' + $targetBranch)
}
else
{
    $possibleSourceBranchPatternsArr = $possibleSourceBranchPatterns.Split(';', [System.StringSplitOptions]::RemoveEmptyEntries)
    
    if ($possibleSourceBranchPatternsArr.Contains($sourceBranchFilter))
    {
        Write-Host ('PR can be allowed for target branch: ' + $targetBranch + ' from source branch: ' + $sourceBranch)
        Write-Host ('Allowing PR validation build to proceed...')
    }
    else
    {
        Write-Warning ('PR cannot be allowed for target branch: ' + $targetBranch + ' from source branch: ' + $sourceBranch)
        Write-Error ('Stopping PR validation build from proceeding...')
    }
}