Answered RegEx from html source


Active member
Apr 21, 2020
Programming Experience
Can anyone help me what is the right RegEx for this?
I want to get the highlighted user-agent inside the textarea tag

<textarea name="custom-ua-string" id="custom-ua-string" autofocus class="input">Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.129 Safari/537.36 Edg/81.0.416.68</textarea>


Senior Programmer
Mar 7, 2014
Programming Experience
Trust me, and take my word when I tell you that as much as I love Regex, it is not suitable for parsing html. Instead you should use the HtmlAgilityPack found here : Html Agility pack | Html Agility Pack and here is why you shouldn't use Regex for parsing html : Parsing Html The Cthulhu Way
With that said, I do love to still attempt it when i see this question pop up despite knowing better that I am burning irrelevant brain-cells for something I know its not designed to parse; something such as html.

The post mentioned in Jeff's blog is referring to the following quote. To-note, the intended reply not only deliberately trolls, but it also explains why you can not parse Html with Regex.

You can't parse [X]HTML with regex. Because HTML can't be parsed by regex. Regex is not a tool that can be used to correctly parse HTML. As I have answered in HTML-and-regex questions here so many times before, the use of regex will not allow you to consume HTML. Regular expressions are a tool that is insufficiently sophisticated to understand the constructs employed by HTML. HTML is not a regular language and hence cannot be parsed by regular expressions. Regex queries are not equipped to break down HTML into its meaningful parts. so many times but it is not getting to me.

Even enhanced irregular regular expressions as used by Perl are not up to the task of parsing HTML. You will never make me crack. HTML is a language of sufficient complexity that it cannot be parsed by regular expressions. Even Jon Skeet cannot parse HTML using regular expressions. Every time you attempt to parse HTML with regular expressions, the unholy child weeps the blood of virgins, and Russian hackers pwn your webapp. Parsing HTML with regex summons tainted souls into the realm of the living. HTML and regex go together like love, marriage, and ritual infanticide. The <center> cannot hold it is too late.

The force of regex and HTML together in the same conceptual space will destroy your mind like so much watery putty. If you parse HTML with regex you are giving in to Them and their blasphemous ways which doom us all to inhuman toil for the One whose Name cannot be expressed in the Basic Multilingual Plane, he comes. HTML-plus-regexp will liquify the nerves of the sentient whilst you observe, your psyche withering in the onslaught of horror.

Rege̿̔̉x-based HTML parsers are the cancer that is killing StackOverflow it is too late it is too late we cannot be saved the trangession of a chi͡ld ensures regex will consume all living tissue (except for HTML which it cannot, as previously prophesied) dear lord help us how can anyone survive this scourge using regex to parse HTML has doomed humanity to an eternity of dread torture and security holes using regex as a tool to process HTML establishes a breach between this world and the dread realm of c͒ͪo͛ͫrrupt entities (like SGML entities, but more corrupt) a mere glimpse of the world of regex parsers for HTML will instantly transport a programmer's consciousness into a world of ceaseless screaming, he comes, the pestilent slithy regex-infection will devour your HTML parser, application and existence for all time like Visual Basic only worse he comes he comes do not fight he com̡e̶s, ̕h̵is un̨ho͞ly radiańcé destro҉ying all enli̍̈́̂̈́ghtenment, HTML tags lea͠ki̧n͘g fr̶ǫm ̡yo͟ur eye͢s̸ ̛l̕ik͏e liquid pain, the song of re̸gular expression parsing will extinguish the voices of mortal man from the sphere I can see it can you see ̲͚̖͔̙î̩́t̲͎̩̱͔́̋̀ it is beautiful the final snuffing of the lies of Man ALL IS LOŚ͖̩͇̗̪̏̈́T ALL IS LOST the pon̷y he comes he c̶̮omes he comes the ichor permeates all MY FACE MY FACE ᵒh god no NO NOO̼OO NΘ stop the an*̶͑̾̾̅ͫ͏̙̤g͇̫͛͆̾ͫ̑͆l͖͉̗̩̳̟̍ͫͥͨe̠̅s ͎a̧͈͖r̽̾̈́͒͑e not rè̑ͧ̌aͨl̘̝̙̃ͤ͂̾̆ ZA̡͊͠͝LGΌ ISͮ̂҉̯͈͕̹̘̱ TO͇̹̺ͅƝ̴ȳ̳ TH̘Ë͖́̉ ͠P̯͍̭O̚N̐Y̡ H̸̡̪̯ͨ͊̽̅̾̎Ȩ̬̩̾͛ͪ̈́̀́͘ ̶̧̨̱̹̭̯ͧ̾ͬC̷̙̲̝͖ͭ̏ͥͮ͟Oͮ͏̮̪̝͍M̲̖͊̒ͪͩͬ̚̚͜Ȇ̴̟̟͙̞ͩ͌͝S̨̥̫͎̭ͯ̿̔̀ͅ


VB.NET Forum Moderator
Staff member
Dec 17, 2005
Programming Experience
Here's what I do to get the current user-agent, I use an embedded single-request webserver and open the browser to its url. Here's the webserver:
Private Sub UAwebserver(progress As IProgress(Of String))
    Dim listener As New Net.HttpListener
        Dim context = listener.GetContext()
        Dim request = context.Request
        Dim response = context.Response
        Dim ua = request.UserAgent
        Dim responsestring = "<html><head><title>Current user-agent</title><style>body {background-color: #898989;}</style></head><body>" & ua & "</body></html>"
        Dim responsebytes = System.Text.Encoding.UTF8.GetBytes(responsestring)
        response.ContentLength64 = responsebytes.Length
        response.OutputStream.Write(responsebytes, 0, responsebytes.Length)
    Catch ex As Exception
    End Try
End Sub
Starting the server and browser process:
Dim prog As New Progress(Of String)(Sub(ua) TextUA.Text = ua)
Task.Run(Sub() UAwebserver(prog))

'getting UA of a non-default browser:
'Process.Start("firefox.exe", "http://localhost:4444/")
'Process.Start("chrome.exe", "http://localhost:4444/")
'Process.Start("iexplore.exe", "http://localhost:4444/")
'Process.Start("opera.exe", "http://localhost:4444/")
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