F# snippet to fetch and return the hash identifier of last commit in the specified repository using regex pattern matching.

Commit.fs

open System.IO
open System.Net
open System.Text.RegularExpressions
open System

/// <summary>
/// Fetches the Id (SHA1 hash) of the most recent GitHub Git commit in the
/// specified repository.
/// </summary>
///
/// <param name="username">The target GitHub username.</param>
/// <param name="repoName">The target repo.</param>
///
/// <param name="shortHash">
/// Specifies if the function should only return the first 7 characters of the
/// hash value rather than the full 41 characters.
/// </param>
///
/// <returns>
/// The most recent commit Id.
/// </returns>
///
/// <remarks>
/// GitHub has a limit of 50 requests/hour for unauthenticated users.
/// See: https://developer.github.com/v3/#rate-limiting
/// </remarks>
let getLastCommit (username: string, repoName: string, shortHash: bool) = 
    let mutable ret = String.Empty

    try
        // GitHub requires TLS 1.2 as of Feb 01 2018
        // .NET Framework 4.5 or up is required to set this protocol
        ServicePointManager.SecurityProtocol <- SecurityProtocolType.Tls12

        let url = sprintf "https://api.github.com/repos/%s/%s/git/refs/heads/master" 
                          username repoName

        let req = WebRequest.Create url :?> HttpWebRequest
        req.Method      <- "GET"
        req.ContentType <- "application/json; charset=utf-8"
        req.UserAgent   <- "Mozilla/5.0 (Windows NT 10.0; Win64; x64) \
                            AppleWebKit/537.36 (KHTML, like Gecko) \
                            Chrome/60.0.3112.113 Safari/537.36"

        use res = req.GetResponse() :?> HttpWebResponse
        if (res.StatusCode = HttpStatusCode.OK) then
            use reader = new StreamReader(res.GetResponseStream())
            let output = reader.ReadToEnd()

            let pattern = "\"sha\"\\s*:\\s*\"((?=[ -~])[^\"]+)\""
            let match' = Regex.Match(output, pattern)

            let commitHash = 
                if match'.Success then
                    let matchVal = match'.Groups.[1].Value
                    if shortHash then matchVal.[0..6] else matchVal
                else
                    "Could not get commit ID. Regex matching wasn't successful."
            ret <- commitHash

    with | :? WebException as ex -> 
        if ex.Status = WebExceptionStatus.ProtocolError && ex.Response <> null then
            // Custom messages for common API exceptions
            let commonEx = 
                use exRes = ex.Response :?> HttpWebResponse
                match exRes.StatusCode with 
                | HttpStatusCode.NotFound  -> "No commits or repository not found."
                | HttpStatusCode.Forbidden -> "Access to information is forbidden."
                | _ -> ex.Message
            ret <- commonEx
        else
            ret <- ex.Message
    ret

Program.fs

[<EntryPoint>]
let main argv = 
    let user, repo = "microsoft", "dotnet"
    let commitHash = getLastCommit(user, repo, false)
    let result = sprintf "Last commit in the repo '%s/%s': %s" user repo commitHash

    Console.WriteLine(result)
    Console.ReadKey() |> ignore
    0