Regular expressions are a powerful language for matching text patterns. This page gives a basic introduction to regular expressions themselves sufficient for our Python exercises and shows how regular expressions work in Python. The Python "re" module provides regular expression support. In Python a regular expression search is typically written as:
match = re.search(pat, str)
The re.search() method takes a regular expression pattern and a string and searches for that pattern within the string. If the search is successful, search() returns a match object or None otherwise. Therefore, the search is usually immediately followed by an if-statement to test if the search succeeded, as shown in the following example which searches for the pattern 'word:' followed by a 3 letter word (details below):
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import re
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def test(match):
if match:
print 'found', match.group() ## 'found word:cat'
else:
print 'did not find'
str = 'an example word:cat!!'
test(re.search(r'word:\w\w\w', str))
The code match = re.search(pat, str) stores the search result in a variable named "match". Then the if-statement tests the match -- if true the search succeeded and match.group() is the matching text (e.g. 'word:cat'). Otherwise if the match is false (None to be more specific), then the search did not succeed, and there is no matching text.
The 'r' at the start of the pattern string designates a python "raw" string which passes through backslashes without change which is very handy for regular expressions (Java needs this feature badly!). I recommend that you always write pattern strings with the 'r' just as a habit.
Joke: what do you call a pig with three eyes? piiig!
The basic rules of regular expression search for a pattern within a string are:
The search proceeds through the string from start to end, stopping at the first match found All of the pattern must be matched, but not all of the string If match = re.search(pat, str) is successful, match is not None and in particular match.group() is the matching text
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## Search for pattern 'iii' in string 'piiig'.
## All of the pattern must match, but it may appear anywhere.
## On success, match.group() is matched text.
match = re.search(r'iii', 'piiig')
test(match)
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match = re.search(r'igs', 'piiig')
test(match)
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## . = any char but \n
match = re.search(r'..g', 'piiig')
test(match)
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## \d = digit char, \w = word char
match = re.search(r'\d\d\d', 'p123g')
test(match)
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match = re.search(r'\w\w\w', '@@abcd!!')
test(match)
Things get more interesting when you use + and * to specify repetition in the pattern
First the search finds the leftmost match for the pattern, and second it tries to use up as much of the string as possible -- i.e. + and go as far as possible (the + and are said to be "greedy").
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## i+ = one or more i's, as many as possible.
match = re.search(r'pi+', 'piiig')# => found, match.group() == "piii"
test(match)
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## Finds the first/leftmost solution, and within it drives the +
## as far as possible (aka 'leftmost and largest').
## In this example, note that it does not get to the second set of i's.
match = re.search(r'i+', 'piigiiii')# => found, match.group() == "ii"
test(match)
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## \s* = zero or more whitespace chars
## Here look for 3 digits, possibly separated by whitespace.
match = re.search(r'\d\s*\d\s*\d', 'xx1 2 3xx')
test(match)
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match = re.search(r'\d\s*\d\s*\d', 'xx12 3xx')
test(match)
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match = re.search(r'\d\s*\d\s*\d', 'xx123xx')
test(match)
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## ^ = matches the start of string, so this fails:
match = re.search(r'^b\w+', 'foobar')
test(match)
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## but without the ^ it succeeds:
match = re.search(r'b\w+', 'foobar')
test(match)
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str = 'purple alice-b@google.com monkey dishwasher'
match = re.search(r'\w+@\w+', str)
test(match)
The search does not get the whole email address in this case because the \w does not match the '-' or '.' in the address. We'll fix this using the regular expression features below.
Square brackets can be used to indicate a set of chars, so [abc] matches 'a' or 'b' or 'c'. The codes \w, \s etc. work inside square brackets too with the one exception that dot (.) just means a literal dot. For the emails problem, the square brackets are an easy way to add '.' and '-' to the set of chars which can appear around the @ with the pattern r'[\w.-]+@[\w.-]+' to get the whole email address:
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match = re.search(r'[\w.-]+@[\w.-]+', str)
test(match)
(More square-bracket features) You can also use a dash to indicate a range, so [a-z] matches all lowercase letters. To use a dash without indicating a range, put the dash last, e.g. [abc-]. An up-hat (^) at the start of a square-bracket set inverts it, so [^ab] means any char except 'a' or 'b'.
The "group" feature of a regular expression allows you to pick out parts of the matching text. Suppose for the emails problem that we want to extract the username and host separately. To do this, add parenthesis ( ) around the username and host in the pattern, like this: r'([\w.-]+)@([\w.-]+)'. In this case, the parenthesis do not change what the pattern will match, instead they establish logical "groups" inside of the match text. On a successful search, match.group(1) is the match text corresponding to the 1st left parenthesis, and match.group(2) is the text corresponding to the 2nd left parenthesis. The plain match.group() is still the whole match text as usual.
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str = 'purple alice-b@google.com monkey dishwasher'
match = re.search('([\w.-]+)@([\w.-]+)', str)
if match:
print match.group() ## 'alice-b@google.com' (the whole match)
print match.group(1) ## 'alice-b' (the username, group 1)
print match.group(2) ## 'google.com' (the host, group 2)
A common workflow with regular expressions is that you write a pattern for the thing you are looking for, adding parenthesis groups to extract the parts you want.
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## Suppose we have a text with many email addresses
str = 'purple alice@google.com, blah monkey bob@abc.com blah dishwasher'
## Here re.findall() returns a list of all the found email strings
emails = re.findall(r'[\w\.-]+@[\w\.-]+', str)
for email in emails:
print email
For files, you may be in the habit of writing a loop to iterate over the lines of the file, and you could then call findall() on each line. Instead, let findall() do the iteration for you -- much better! Just feed the whole file text into findall() and let it return a list of all the matches in a single step (recall that f.read() returns the whole text of a file in a single string):
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# Open file
f = open('data/poem.txt', 'r')
# Feed the file text into findall(); it returns a list of all the found strings
strings = re.findall(r'light\w*', f.read())
strings
The parenthesis ( ) group mechanism can be combined with findall(). If the pattern includes 2 or more parenthesis groups, then instead of returning a list of strings, findall() returns a list of tuples. Each tuple represents one match of the pattern, and inside the tuple is the group(1), group(2) .. data. So if 2 parenthesis groups are added to the email pattern, then findall() returns a list of tuples, each length 2 containing the username and host, e.g. ('alice', 'google.com').
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str = 'purple alice@google.com, blah monkey bob@abc.com blah dishwasher'
tuples = re.findall(r'([\w\.-]+)@([\w\.-]+)', str)
print tuples
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for tuple in tuples:
print tuple[0] ## username
print tuple[1] ## host
Once you have the list of tuples, you can loop over it to do some computation for each tuple. If the pattern includes no parenthesis, then findall() returns a list of found strings as in earlier examples. If the pattern includes a single set of parenthesis, then findall() returns a list of strings corresponding to that single group. (Obscure optional feature: Sometimes you have paren ( ) groupings in the pattern, but which you do not want to extract. In that case, write the parens with a ?: at the start, e.g. (?: ) and that left paren will not count as a group result.)
Regular expression patterns pack a lot of meaning into just a few characters , but they are so dense, you can spend a lot of time debugging your patterns. Set up your runtime so you can run a pattern and print what it matches easily, for example by running it on a small test text and printing the result of findall(). If the pattern matches nothing, try weakening the pattern, removing parts of it so you get too many matches. When it's matching nothing, you can't make any progress since there's nothing concrete to look at. Once it's matching too much, then you can work on tightening it up incrementally to hit just what you want.
The re functions take options to modify the behavior of the pattern match. The option flag is added as an extra argument to the search() or findall() etc., e.g. re.search(pat, str, re.IGNORECASE).
Suppose you have text with tags in it: <b>foo</b> and <i>so on</i>
Suppose you are trying to match each tag with the pattern '(<.*>)' -- what does it match first?
The result is a little surprising, but the greedy aspect of the .* causes it to match the whole <b>foo</b> and <i>so on</i> as one big match. The problem is that the .* goes as far as is it can, instead of stopping at the first > (aka it is "greedy").
There is an extension to regular expression where you add a ? at the end, such as .*? or .+?, changing them to be non-greedy. Now they stop as soon as they can. So the pattern (<.*?>) will get just <b> as the first match, and </b> as the second match, and so on getting each <..> pair in turn. The style is typically that you use a .*?, and then immediately its right look for some concrete marker (> in this case) that forces the end of the .*? run.
The *? extension originated in Perl, and regular expressions that include Perl's extensions are known as Perl Compatible Regular Expressions -- pcre. Python includes pcre support. Many command line utils etc. have a flag where they accept pcre patterns.
An older but widely used technique to code this idea of "all of these chars except stopping at X" uses the square-bracket style. For the above you could write the pattern, but instead of .* to get all the chars, use [^>]* which skips over all characters which are not > (the leading ^ "inverts" the square bracket set, so it matches any char not in the brackets).
The re.sub(pat, replacement, str) function searches for all the instances of pattern in the given string, and replaces them. The replacement string can include '\1', '\2' which refer to the text from group(1), group(2), and so on from the original matching text.
Here's an example which searches for all the email addresses, and changes them to keep the user (\1) but have yo-yo-dyne.com as the host.
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str = 'purple alice@google.com, blah monkey bob@abc.com blah dishwasher'
re.sub(r'([\w\.-]+)@([\w\.-]+)', r'\1@yo-yo-dyne.com', str)
For practice with regular expressions, go to the notebook 6.5 - Baby names
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Note: This notebook is based on Google's python tutorial https://developers.google.com/edu/python
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