Well That Was Silly Of Me, Issues with Sed….

Refreshing my memory on sed caused me to run into two issues tonight. Firstly…. the -n parameter only shows the patterns that you wish to show [after it is used]. Secondly, the order of deleting and printing lines  matters. It turns out that it matters a lot.

Lets say you have a file named contents. It contains:

Gooooogle Pictures
Google Plus

Let’s assume that you wanted to just show all lines that contained “Gooogle” [and its similar brothers] with sed. You would write a line that contains this:

sed -e '/Goo[o]\+gle/p' content

Right? Nope. It’ll show all of the items, despite that you used the print command to display items that matched that pattern. To fix this, put in the -n option before -e.

That’s great… But that returns: Gooogle, GooooogleBot, and Gooooogle Pictures. In this example, we don’t like GoogleBot. So lets remove it. You may now write something like: 

sed -n -e '/Goo[o]\+gle/p' -e '/Goo[o]\+gleBot/d' content

It seems like a logical extension. Right? The next regular expression should pass over the printed lines left and make an evaluation. Nope, it doesn’t. It’ll display the same results prior to the second expression. What’s going on? Its not a bad expression. Its not a bad command. It’s due to the placement of the prints, deletes, and where you ask that the pattern space be shown. This is some odd quirk, that I haven’t found an explanation for [yet]. But what it turns out to be the correct way of doing it is to rearrange everything where the deletes are first, and then the prints occur [Also, to refuse to print the pattern space after the deletes (weird I know).

So the correct form is:

sed -e '/Goo[o]\+gleBot/d' -n -e '/Goo[o]\+gle/p' content

Bizarre? Yes, very much so, but it works.

Regular Expressions Tester

This is one elusive tool that has been bugging me for years. There are quite a lot of regular expression tools out there, and not all of them test the expression environment that you need. [Rarely is it ever mentioned which environment its testing for either]. Through the magic of google I’ve found a tool to test regular expressions in Java. Its RegExPlanet. The tool allows you to test regular expressions for the environments: Java, Ruby, PHP, .NET, and Python.

For right now, all I can attest to is its ability to test regular expressions in Java. It goes beyond a simple regular expression match.

For Java Regular Expressions, It’ll show:

  • Group Counts
  • Individual Group Matches 
  • The Java String [to copy and paste directly into your code] (However, at the time of writing it does not allow you to enter the RegEx in the Java String format)
  • Replace First [if there is a replacement string]
  • Replace All [if there is a replacement string]
  • Simple Validation

From the looks of it, it also has a feature to save your previous expressions. That’s really cool! It’s practically the Fiddle [see jsFiddle] of Regular Expressions. It hints that it has the ability to save them to shortcodes (to share), but I have not tried that feature. Additionally, from the the looks of it; this is a new site. I would encourage developers to use this site.