So far we have been reading through files, looking for patterns and extracting various bits of lines that we find interesting. We have been using string methods like split and find and using lists and string slicing to extract portions of the lines.
This task of searching and extracting is so common that Python has a very powerful library called regular expressions that handles many of these tasks quite elegantly. The reason we have not introduced regular expressions earlier in the book is because while they are very powerful, they are a little complicated and their syntax takes some getting used to.
Regular expressions are almost their own little programming language for searching and parsing strings. As a matter of fact, entire books have been written on the topic of regular expressions. In this chapter, we will only cover the basics of regular expressions. For more detail on regular expressions, see:
The regular expression library must be imported into your program before you can use it. The simplest use of the regular expression library is the search() function.
The following program demonstrates a trivial use of the search function.
import rehand = open('mbox-short.txt')for line in hand: line = line.rstrip() if re.search('From:', line) : print line
We open the file, loop through each line and use the regular expression search() to only print out lines that contain the string “From:”. This program does not use the real power of regular expressions since we could have just as easily used line.find() to accomplish the same result.
The power of the regular expressions comes when we add to special characters to the search string that allow us to more precisely control which lines match the string. Adding these special characters to our regular expression allow us to do sophisticated matching and extraction while writing very little code.
For example, the caret character is uses in regular expressions to match “the beginning” of a line. We could change our application to only match lines where “From:” was at the beginning of the line as follows:
import rehand = open('mbox-short.txt')for line in hand: line = line.rstrip() if re.search('ˆFrom:', line) : print line
Now we will only match lines that start with the string “From:”. This is still a very simple example that we could have done equivalently with the startswith() method from the string library. But it serves to introduce the notion that regular expressions contain special action characters that give us more control as to what will match the regular expression.