Learning ColdFusion 8: REMatch() For Regular Expression Matching

Posted June 13, 2007 at 8:21 AM

Tags: ColdFusion

ColdFusion has been awesome at finding regular expressions in strings with REFind() and REFindNoCase() and it has been easy to alter strings with regular expressions using REReplace() and REReplaceNoCase(). But, until now, ColdFusion has been extremely weak when it came to retrieving substrings from a chunk of text using regular expressions. Thankfully, ColdFusion 8 has introduced REMatch() and REMatchNoCase(). REMatch() searches through a string using a regular expression and returns all matching patterns in an array.

To demonstrate the power and ease of use of this new regular expression matching function, let's grab some search results from Google.com and then from that content, let's grab out the titles and links using a regular expression:

 Launch code in new window » Download code as text file »

Here, we are using the combination of H2 and A tags to create our search results pattern. Running the above code, we get the following CFDump output:

How easy was that?!?

If you have not performed this action before, you might not even realize what kind of a gold mine this is. Let's compare the REMatch() function to how something like this would have been accomplished in pre-ColdFusion-8 days. One way would be to keep looping over REFind() using the Len and Pos arrays... but honestly, that is the most ghetto of all possible solutions and it is shameful that ColdFusion even wanted people to do that (no offense if that's how you do it). So, instead, I will compare this to the ColdFusion / Java equivalent.

ColdFusion's Java libraries have regular expression Patterns libraries that allow us to loop over patterns in a string:

 Launch code in new window » Download code as text file »

As you can see, this is not too complicated, but it is much more code than the above REMatch() method. And, just so you can see that these are doing the exact same thing, when you run the code above, you get the following CFDump output:

If you are used to doing things like this (the above), then REMatch() is going to make your life a lot easier. Of course, REMatch() is no silver bullet. ColdFusion 8 and REMatch() still use the same regular expression engine that was used in earlier versions of ColdFusion. This means that the regular expressions used in REMatch() are still more limited than those used in the Java Pattern / Matcher solution.

In our above examples, we are grabbing the H2 tags as well as the links within them. We are doing this so that we don't just grab random links off the page (only the search results have that H2 tag with that class). But let's say we didn't want to retrieve the H2 tags, we just wanted to make sure they existed in our pattern. To achieve this, we could use positive look behinds and positive look aheads in our pattern:

(?i)(?<=<h2 class=r>)<a[^>]+>.+?</a>(?=</h2>)

Here, we are saying that the H2 tags have to start the pattern and end the pattern, however, we do NOT want to grab them. This is purely for assertive matching. This is a valid regular expression, but if you tried to run this in the REMatch() function it will throw a ColdFusion error about ill-formed regular expressions. ColdFusion 8, using the older regular expression engine, still cannot handle look-behinds.

If you are dead-set on using them, you can, of course still use the Java Pattern / Matcher to grab the results. If we modified our Java Pattern / Matcher example above to use our new regular expression with look ahead/behinds, we would get the following CFDump output:

Notice that, now, our search results only have the A tags. The H2 tags, used in the ahead/behind assertive matching, were not captured in our results group.

REMatchNoCase() does the same thing as REMatch() only it does not have case sensitivity. However, you can build case-insensitivity directly into your regular expression using the flag (?i). Notice that all of my regular expressions above start with this flag - that makes them case-insensitive. Running:

 Launch code in new window » Download code as text file »

... would be the same thing as running:

 Launch code in new window » Download code as text file »

Notice that I have changed the method from REMatch() to REMatchNoCase() and removed the (?i) flag.

So, while REMatch() and REMatchNoCase() are not the cure-all for your regular expression matching, I am sure you can see that for less-complex patterns, these new ColdFusion 8 functions are going to make your life a lot more cozy. One thing that I think these functions lacks is a scope. When you use REReplace(), you can tell it to replace ONE instance of the pattern or ALL instances of the pattern. This seems like something that REMatch() and REMatchNoCase() should have an optional third argument (defaulting to ALL). I can imagine situations where you know you might have multiple pattern matches, but for optimization, you only care about the first one and you want ColdFusion to stop matching after the first match. But oh well, maybe in ColdFusion 9 :)

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Reader Comments

Another not necessary but a good idea would be to send a user-agent string along with your http request so your script looks like a real browser.

@Dustin,

Good point... I am actually sending the user agent as part of the CFHttp tag itself (#CGI.http_user_agent#), however this only works because I am testing in a browser. If this was launched via a scheduled task, I believe that shows up as "ColdFusion" user agent. Therefore, it is a good idea to actually put in an explicit user agent.

Additionally, this value can be done as a CFHttpParam:

<cfhttpparam
type="HEADER"
name="user-agent"
value="Mozilla......."
/>

Thanks for pointing that out.

This function is fantastic-- thanks for blogging it, Ben! Just this Monday, as I was writing code using ReFindNoCase() and looping over the string to find each new match, I was wishing that CF would just have some function to return all of the matches across the whole string. Now I know it's there. Thank goodness-- it will save a lot of work!

@Tom,

Yeah looping through in indexes is just so ganky and painful (I know because that's how I did it for a long, long time). If you are feeling adventurous, check out the Java-ish method above. Its a bit strange at first, but once you get going with it, it is AWESOME. Plus, the Matcher.Group() can take arguments to actually give you individual group matches. So, for example, Matcher.Group( 2 ) will return the second captured group.

Of course, if Java is not your thing, ColdFusion 8 should hopefully be here soon :D

"these functions lack a scope"

What!? That's just retarded. It's extremely common to only want the first match. And the scope should default to "one", not "all", so that it would be like REReplace, as well as every other programming language's default regular expression matching construct that I'm aware of.

@Steve,

I agree. If you only know you want the first match, or even more, if you know there is ONLY one match, you would be able to save a lot of processing by having it halt the moment it finds that match.

Check it out:

http://www.forta.com/blog/index.cfm/2007/5/4/Scorpio-Adds-Two-New-RegEx-Functions

Before Forta even had a chance to explain how the function worked, I took a guess... what's that third argument? That's right, a scope. Oh well, dare to dream :)

On a different subject, I think that there may be something wrong with the page's syntax coloring (I'm viewing in IE7) - I can see all the text down the entry has gone navy, as though a tag is not closed...

@Shuns,

Thanks for pointing that out. It looks like the REMatch() highlighting is getting messed up for some reason. Notice that the (?i) flag is a different color than the text next to it (which is should not be). I will check this first thing in the morning.

I think attributes that have HTML tags in the (ex. "<h2>") confuse the algorithm.

Yeah thats what I was thinking :P

@Shuns,

I have updated my color-coding algorithm. Can you take a quick look at this post and make sure the color coding is not breaking any more. It looks good to me, but I'd like a second pair of eyes.

Thanks.

Yeah looks good to me, nice job.

BTW keep up the good work - alot of very good info on your site :D

Awesome dude, glad you enjoy it. Thanks for the heads up on the coloring issues. Hopefully that should be the last time.

Ben,

Will this function work with re lookarounds?

Thanks,
Will

@Will,

The underlying regular expression engine being used is still the same. So, I believe that it will handle POSITIVE look ahead/behinds. But, I think it still doesn't handle NEGATIVE look ahead/behinds. Definitely not negative look-behinds. Maybe nothing behind positive or negative. I've been using the Java regex for anything complicated, so I can't really remember.

Ben,

you write: Let's compare the REMatch() function to how something like this would have been accomplished in pre-ColdFusion-8 days. In that sample you use the following code: <cfset arrTitles = [] />
which happens not to work in CF 7 - bad example! ;-)

Better use:
<cfset arrTitles = ArrayNew(1) />

Best wishes,
Bard

@Bard,

Good catch ;)

Fantastic post.

Saved me hours of work.

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