Processing regular expressions

About regular expressions

As an extension to the find and replace functions described in String operations and functions, Jython supports regular expressions. Regular expressions (RE) are strings that contain plain match text and control characters and provide an extremely powerful string search and replace facility. Jython supports (at least) the following forms of regular expressions:

• re module is a built-in part of Jython.
• Java works if you're running Jython on Java 1.4 or above.
• Apache ORO works if you add the ORO package to your CLASSPATH.

Regular expression formats

The simplest RE is an exact string to match. More complex REs include special control characters. The control characters allow you to create patterns of matching strings. For more information on RE syntax and options see Appendix H: Regular expression control characters and the Python Library Reference.

[Jennette, from Barry: We need to get the spacing right in this table, I have multiple nbsp's that show as only one space.]-->

Below are some example REs and the strings they match:

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Regular expressions functions

The Jython re module provides support for regular expressions. re's primary functions are findall, match, and search to find strings, and sub and subn to edit them. The match function looks at the start of a string, the search function looks anywhere in a string, and the findall function repeats search for each possible match in the string. search is (by far) the most used of the regular expression functions.

Here are some of the most common RE functions:

Note that the matching functions return None if no match is found. Otherwise the match functions will return a Match object from which details of the match can be found. See the Python Library Reference for more information on Match objects.

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Two function examples

Let's take a look at some examples of regular expressions functions in action:

import re

# do a fancy string match
if re.search(r"^\s*barry\s+feigenbaum\s*$", name, re.I):
   print "It's Barry alright"

# replace the first name with an initial
name2 = re.sub(r"(B|b)arry", "B.", name)

If you are going to use the same pattern repeatedly, such as in a loop, you can speed up execution by using the compile function to compile the regular expression into a Pattern object and then using that object's methods, as shown here:

import re
patstr = r"\s*abc\s*"
pat = re.compile(patstr)
# print all lines matching patstr
for s in stringList:
    if pat.match(s, re.I): print "%r matches %r" % (s, patstr)

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Regular expression example: Grep

The following simplified version of the Grep utility (from grep.py) offers a more complete example of a Jython string function.

""" A simplified form of Grep. """

import sys, re

if len(sys.argv) != 3:
    print "Usage: jython grep.py <pattern> <file>"
else:
    # process the arguments
    pgm, patstr, filestr = sys.argv
    print "Grep - pattern: %r file: %s" % (patstr, filestr)
    pat = re.compile(patstr)  # prepare the pattern


                        # see File I/O in Jython
                     for more information
    file = open(filestr)      # access file for read
    lines = file.readlines()  # get the file
    file.close()

    count = 0
    # process each line
    for line in lines:
        match = pat.search(line)    # try a match
        if match:                   # got a match
            print line
            print "Matching groups: " + str(match.groups())
            count += 1
    print "%i match(es)" % count

When run on the words.txt file from File I/O in Jython , the program produces the following result:

C:\Articles>jython grep.py "(\w*)!" words.txt
Grep - pattern: '(\\w*)!' file: words.txt
How many times must I say it; Again! again! and again!

Matched on: ('Again',)
Singing in the rain! I'm singing in the rain! \
    Just singing, just singing, in the rain!

Matched on: ('rain',)
2 match(es)

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Table of contents

• About this tutorial
• Object-oriented programming in Jython
• Advanced object-oriented programming
• Debugging Jython
• Java support in Jython
• Java thread support in Jython
• Interfacing with Java services
• Jython string processing
• Processing regular expressions
• File I/O in Jython
• A simple Swing GUI
• Wrap-up
• Appendices
• Resources
• About the author

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