How to create Gmail filter searching for text only at start of subject line?

Using Google Apps Script, you can use this function to filter email threads by a given regex:

function processInboxEmailSubjects() {
  var threads = GmailApp.getInboxThreads();
  for (var i = 0; i < threads.length; i++) {
    var subject = threads[i].getFirstMessageSubject();
    const regex = /^\[RELEASE\]/; //change this to whatever regex you want, this one should cover OP's scenario
    let isAtLeast40 = regex.test(subject)
    if (isAtLeast40) {
      Logger.log(subject);

      // Now do what you want to do with the email thread. For example, skip inbox and add an already existing label, like so:
      threads[i].moveToArchive().addLabel("customLabel")
    }
  }
}

As far as I know, unfortunately there isn't a way to trigger this with every new incoming email, so you have to create a time trigger like so (feel free to change it to whatever interval you think best):

function createTrigger(){ //you only need to run this once, then the trigger executes the function every hour in perpetuity
  ScriptApp.newTrigger('processInboxEmailSubjects').timeBased().everyHours(1).create();
} 

Regex is not on the list of search features, and it was on (more or less, as Better message search functionality (i.e. Wildcard and partial word search)) the list of pre-canned feature requests, so the answer is "you cannot do this via the Gmail web UI" :-(

There are no current Labs features which offer this. SIEVE filters would be another way to do this, that too was not supported, there seems to no longer be any definitive statement on SIEVE support in the Gmail help.

Updated for link rot The pre-canned list of feature requests was, er canned, the original is on archive.org dated 2012, now you just get redirected to a dumbed down page telling you how to give feedback. Lack of SIEVE support was covered in answer 78761 Does Gmail support all IMAP features?, since some time in 2015 that answer silently redirects to the answer about IMAP client configuration, archive.org has a copy dated 2014.

With the current search facility brackets of any form () {} [] are used for grouping, they have no observable effect if there's just one term within. Using (aaa|bbb) and [aaa|bbb] are equivalent and will both find words aaa or bbb. Most other punctuation characters, including \, are treated as a space or a word-separator, + - : and " do have special meaning though, see the help.

As of 2016, only the form "{term1 term2}" is documented for this, and is equivalent to the search "term1 OR term2".

You can do regex searches on your mailbox (within limits) programmatically via Google docs: http://www.labnol.org/internet/advanced-gmail-search/21623/ has source showing how it can be done (copy the document, then Tools > Script Editor to get the complete source).

You could also do this via IMAP as described here: Python IMAP search for partial subject and script something to move messages to different folder. The IMAP SEARCH verb only supports substrings, not regex (Gmail search is further limited to complete words, not substrings), further processing of the matches to apply a regex would be needed.

For completeness, one last workaround is: Gmail supports plus addressing, if you can change the destination address to [email protected] it will still be sent to your mailbox where you can filter by recipient address. Make sure to filter using the full email address to:[email protected]. This is of course more or less the same thing as setting up a dedicated Gmail address for this purpose :-)

The only option I have found to do this is find some exact wording and put that under the "Has the words" option. Its not the best option, but it works.