I’ll break it down to the individual sections with a brief explaination.

//protocol and subdomain
(http(s)?:\/\/)?(([a-z]+[a-z0-9\-]*[.])?

The first part includes the protocol (http:// or https://), I am only dealing with web http urls here and it is optional in my app hence the ? at the end of the first group, the rest includes an optional subdomain which should start with one or more letters followed by zero or more letters/numbers/hyphens and a dot. This first subdomain and dot is also optional. So far this would match: [empty string] http:// https//www. https://ww2 etc.

//server hostname
([a-z0-9]+[a-z0-9\-]*[.])+[a-z]{2,3}|localhost)

This next part includes the rest of the web host, the first grouping (first enclosing brackets) specifies the start of the hostname or a further subdomain which must start with a letter or number followed by a dot (the dot as a character set is how to represent the dot in Flex, you might be able to just use \.). This can be repeated many times, but then should be followed by 2 or three characters. Alternatively the hostname localhost can be used instead, the extra closing bracket matches the additional opening one after the protocol. This section should match: www.example.com localhost example.com example.co.uk co.uk etc.

//web path
(\/[a-z0-9_-]+[a-z0-9_ -]*)*\/?

This next part consists of the optional path (directory from the web root), it starts with a forward slash and can be any number of letters, numbers, underscores, spaces or hyphens, but can not start with a space (you might need to backslash escape your hypen in a different language. The trailing backslash is also optional as is the entire path. This part should match: [empty string] / /directory /a/b/ etc.

//query string
(\\?[a-z0-9_-]+=[a-z0-9 ',.-]*(&[a-z0-9_-]+=[a-z0-9 ',.-]*)*)?

This part contains the optional query string part of the URL. Starting with a ? (may require only a single backslash in a different language), followed by the first parameter made up of one or more letters/numbers/underscores/hyphens the equals sign, followed by an optional parameter value made up of letters/numbers/spaces/apostrophes/commas/dots/hypens. This parameter=value part of the query string can be repeated several times after that but each extra parameter should be preceded with the ampersand (you would normally use just the & for this, but flex requires the encoded version). This section could match: [empty string] ?a= ?a=bc ?a=b&c=d&e=f etc.

//fragment
(#[a-z0-9/_-]*)?$

Finally the last part of the expression contains the optional url fragment (the part with a #). In my case I specified zero or more letters/numbers/forward slashes/underscores/hypens (flex does not require escaping the forward slash when it is included in a character set). Then the dollar sign specifies that there should not be anything else after this. This could match: [empty string] # #value #a/b/c etc.

I hope this is useful to those struggling to create their own URL regular expression matchers. Flex devs remember to double escape the ? for the query string part of the URL.

• http://www.youtube.com/watch?v=[videoid]
• http://www.youtube.com/v/[videoid]
• http://www.youtube.com/[userchannel]/[somepath]/[videoid]

Sounds like a job for a regular expression right? Since we also wanted to do some validation on the field to make sure we were given a valid YouTube video url or video id, it seemed that the RegExpValidator class was perfect for the job. However after adding the RegExpValidator in MXML I had a problem that took a good while to figure out what was wrong.

Below is the snippet of code containing the regular expression string, its use in the validator and a validation handler function, apologies for the wrapping of the regular expression, I have split it below where the main formats are separated by pipes.

The problem was, nothing was coming out as valid, not even the videoid just by itself. So wrote a test using the regular expression, creating a RegExp object using the expression and testing some urls using the exec function and it appeared to be working fine, so why not with the validator? In the end it turned out to be that the problem was down to several characters needing to be escaped. The correct expression that worked is below:

Since the regular expression used in RegExpValidator needs to be a String and the String is used as a bound property in MXML the curly brackets need to be be preceded with a backslash, because without them in MXML it means a data binding. That didn’t make much sense to me, since I was binding a String variable which contained the expression, but ok, I can kind of see the problem. What made even less sense was that I needed to double escape the ? when I actually wanted to include a literal ? in the expression. So I suppose since it is a string and I want to include the backslash character before the question mark I need to escape the backslash itself, leaving \\?, but then shouldn’t I have to do that for all the other times I need to include a backslash in the expression? Well it turns out the answer is no, I do not really understand it and only figured this out after a lot of debugging.

Another pitfall to avoid, if you are binding on a condition in MXML and you need to use logical operators, you will need to encode the logical AND && should be &amp;&amp;, the less than < should be &lt; or greater than signs > should be &gt;, logical OR || works fine. However in those cases you should really be binding to a function which returns the boolean result you are looking for.

Working with MXML is cool because you can create data-bindable UI components quickly and easily, but it really isn’t when you have to worry about silly issues like wondering why your regular expressions are not working.

Recently I had to extract some data which was stored as xml in a field within a mysql database table. To explain briefly, this data was actually saved as part of an XMPP (jabber) chat message by an openfire server. Openfire is a real time collaboration server which includes jabber functionality. To store the chat history Openfire records the chat message as an xml chunk in a field named body in a particular database table ofMucConversationLog. I needed to extract certain data that our system had communicated as a chat message through openfire, to do this I used a combination of regular expressions and string manipulation.

Ok, lets say I have messages that are either in format A or B as below:

<messageformata>
  <child1>some text</child1>
  <child2>some more text</child2>
</messageformata>

<messageformatb>
  <child1>some text</child1>
  <child2>some more text</child2>
</messageformatb>

Each row in the database can contain xml in the format of A or B and there are many rows. If I need to extract only the contents of the first child of message format a, I could use the following SQL code to do it (assuming I’ve created a temporary table called temptable to store these values):

Unfortunately the REGEXP operator can only be used for testing true or false and so is probably only useful in a WHERE clause. There are no capturing options like in other languages (php, perl java etc.), however you can use it in combination with the substring and locate functions to get the job done, although it is admittedly tedious.