Introducing Regular Expressions

Chapter 10. The End of the Beginning

“Unix was not designed to stop you from doing stupid things, because that would also stop you from doing clever things.” —Doug Gwyn

Congratulations for making it this far. You’re not a regular expression novice anymore. You have been introduced to the most commonly used regular expression syntax. And it will open a lot of possibilities up to you in your work as a programmer.

Learning regular expressions has saved me a lot of time. Let me give you an example.

I use a lot of XSLT at work, and often I have to analyze the tags that exist in a group of XML files.

I showed you part of this in the last chapter, but here is a long one-liner that takes a list of tag names from lorem.dita and converts it into a simple XSLT stylesheet:

grep -Eo '<[_a-zA-Z][^>]*>' lorem.dita | sort | uniq | sed '1 i\
<xml:stylsheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">\

; s/^</\
<xsl:template match="/;s/ id=\".*\"//;s/>$/">\
 <xsl:apply-templates\/>\
<\/xsl:template>/;$ a\
\
</xsl:stylesheet>\
'

I know this script may appear a bit acrobatic, but after you work with this stuff for a long time, you start thinking like this. I am not even going to explain what I’ve done here, because I am sure you can figure it out on your own now.

Here is what the output looks like:

<xml:stylsheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:template match="body"> <xsl:apply-templates/> </xsl:template> <xsl:template match="li"> <xsl:apply-templates/> ...