function Set-EnvBuildNumber {
    param(
        [AllowEmptyString()][string]$Content,
        [Parameter(Mandatory = $true)][string]$BuildNumber
    )
    if ($BuildNumber -notmatch '^\d+$') { throw 'Build number must contain digits only.' }
    $newline = if ($Content.Contains("`r`n")) { "`r`n" } else { "`n" }
    $remaining = [regex]::Replace($Content, '(?m)^[\t ]*(?:export[\t ]+)?BUILD_NUMBER[\t ]*=[^\r\n]*(?:\r?\n|$)', '')
    return "BUILD_NUMBER=$BuildNumber$newline$remaining"
}