12.1. Regular Expressions

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:

https://en.wikipedia.org/wiki/Regular_expression

https://docs.python.org/library/re.html

The regular expression library re 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.

This code uses regular expressions to print lines that contain “From:”.

import re hand = 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.

11-9-2: Which of the following correctly imports the regex library?

• import regex
• The module name is not regex.
• import re.search()
• We are not importing a specific regex function.
• import re
• Correct! This is the proper way to import the regular expression library.
• import regular_expressions
• This is not the module name. It is much shorter.

The power of the regular expressions comes when we add 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 used in regular expressions to match “the beginning” of a line. We could change our program to only match lines where “From:” was at the beginning of the line as follows:

Using regular expressions to print lines that contain “From:” at the beginning of the line.

import re hand = open('mbox-short.txt') for line in hand: line = line.rstrip() if re.search('^From:', 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.

11-9-4: Which lines will the following code print?

import re
hand = open('mbox-short.txt')
for line in hand:
    line = line.rstrip()
    if re.search('b', line):
        print(line)

• Any line that contains a 'B'
• Regular expressions are case sensitive for individual characters.
• Any line containing a 'b'
• Correct! Since there is no ^ before the 'b', we are only looking at lines that contain the letter 'b'.
• Lines that start with the letter 'b'
• Since there is no ^, we are only looking for lines that contain 'b', not those that start with 'b'.
• Lines that contain 'B' or 'b'
• Regular expressions are case sensitive for individual characters, so it will only look for 'b'.

Construct a block of code that loops reading lines from a file, removes whitespace on the right of the line, and prints out any line starting with ‘Wolverines’.

        import re
---
with open('mbox-short.txt') as fhand:
---
    for line in fhand:
---
        line = line.rstrip()
---
        line = line.split() #paired

---
        if re.search('^Wolverines', line):
---
        if re.search('Wolverines', line): #paired
---
            print(line)
        

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