12.14. Group Work: Regular Expressions (Regex)

It is best to use a POGIL approach with the following. In POGIL students work in groups on activities and each member has an assigned role. For more information see https://cspogil.org/Home.

Note

If you work in a group, have only one member of the group fill in the answers on this page. You will be able to share your answers with the group at the bottom of the page.

Learning Objectives

Students will know and be able to do the following.

Content Objectives:

• Learn about search and findall and what they return

• Learn about some common quantifiers (*, +, ?, {n})

• Learn about character classes (\d, \w, \s, .)

• Learn about character sets ([an]) and ranges ([0-9])

• Learn how to negate a character set using [^0-9]

• Learn how to escape a special character to match it

• Learn about greedy matching and how to make it not greedy

• Learn how to return just part of a match using parentheses

12.14.1. Regex Methods

Two of the methods that you can use with regular expressions are search and findall. Note that you must import re to use these.

Run the code below to see what it prints.

import re str = "The rain in Spain" # try the search method x = re.search("ai", str) print(x) # show what happens if you check the result if x: print(True) else: print(False) # try the search method again x = re.search("blue", str) # show what happens if you check the result print(x) if x: print(True) else: print(False) # try the findall method y = re.findall("ai", str) print(y) y = re.findall("blue", str) print(y)

12.14.2. Quantifiers

You can specify how many items to match using quantifiers. They refer to the item to their left. The quantifiers are ?, +, *, {n}, and {n,m}.

Run the code below to see what it prints.

import re str1 = "ab abc abcc abcccc abbccc aabbcc" res1 = re.findall("abc+", str1) print(res1) str2 = "color colour colouur" res2 = re.findall("colou?r", str2) print(res2) str3 = "b bo boo booo booooo" res3 = re.findall("bo*", str3) print(res3) str4 = "ab abc abcc abcccc abbccc aabbcc" res4 = re.findall("abc{2}", str4) print(res4) str5 = "ab abc abcc abcccc abbccc aabbcc" res5 = re.findall("abc{1,3}", str5) print(res5)

11-9-6: How many c’s must there be in a row for c{2} to match at least part of the string?

• 0 to many
• No, this would be 'c*'
• 0 to 2
• No, this would be just 'c'
• exactly 2
• No, it will match strings that have more than 2 c's in a row.
• 2 or more
• This will match 2 c's but there can be more in the string.

11-9-8: Drag each symbol to the number of items it matches. Look at the code above.
• ?
• Zero to one
• *
• Zero to many
• +
• One to many
• {2}
• Two
• {1,3}
• One, two, or three

12.14.3. Character Sets

You can use [] to specify that you need to match any one item in the [].

Run the code below to see what it prints.

import re str1 = "ben bean been ban bottom" res1 = re.findall("b[ea]n", str1) print(res1)

11-9-10: What does [ea] mean?

• Match either an 'e' or 'a' one time
• It will match one of the items listed in []
• Match 'ae' one time
• It will match one of the items listed in []
• Match either an 'e' or 'a' one to many times
• This would be true if it was [ae]+
• Match 'ae' one to many times
• This would be true if it was (ae)+

12.14.4. Character Ranges

You can specify a range of items to match.

Run the code below to see what it prints.

import re str1 = "832 3928.23 382.28378 5 92,000 32-928 +32 -32 ABC" res1 = re.findall("[0-9.]+", str1) print(res1) res2 = re.findall("[^0-9.]+", str1) print(res2)

11-9-12: What does [0-9.]+ mean?

• Match any digit or period one or more times. Special characters like '.' just match themselves in [].
• Items in the [] match themselves and are not treated as special characters other than '-'
• Match any digit or anything that isn't a new line one or more times
• The period in a [] just means match a period
• Match any digit or period zero to many times
• The + outside of the [] means match one or more
• Match any digit or anything that isn't a new line zero to many times
• The period in a [] just means match a period and the + means match one or more times

11-9-13: What does [^0-9.]+ mean?

• Match anything other than 0-9 and a period zero to one times
• The + means one to many times
• Match anything other than 0-9 and a period one to many times
• Correct!
• Match ^ or 0-9 or a period zero to one times
• The ^ negates the items
• Match ^ or 0-9 or a period one to many times
• The ^ negates the items

12.14.5. Character Classes

Run the code below to see what it prints.

import re str1 = "mat met m3t m!t m t mitten" res1 = re.findall("m.t", str1) print(res1) str2 = "barbarer5@umich.edu uche@umich.edu lucy@umich.edu" res2 = re.findall("\w+@\w+", str2) print(res2) str3 = "mat met m3t m!t mitten" res3 = re.findall("\sm.t\s", str3) print(res3) str4 = "0013 23 093 000029 320 888" res4 = re.findall("0*\d\d", str4) print(res4)

11-9-15: Drag each item to what it matches Look at the code above.
• .
• Any single character other than a newline
• \d
• A digit (0-9)
• \w
• A word character which is alphanumeric plus underscore
• \s
• A whitespace character (including space, tab, and newline)

Run the code below to see what it prints.

import re str1 = 'From: stephen.marquard@uct.ac.za Sat Jan walk@12 5 09:14:16 2008' res1 = re.findall('\S+@\S+', str1) print(res1) str2 = "What? Come here, 24601!" res2 = re.findall("\W+", str2) print(res2) res3 = re.findall("\D+", str2) print(res3)

11-9-17: Drag each item to what it matches Look at the code above.
• \W
• Any non-word character (not alphanumeric or underscore)
• \S
• Any non-whitespace character (not space, tab, or newline)
• \D
• Any non-digit character (not 0-9).

12.14.6. Escaping Special Characters

If you want to match something that is normally a special character in regex you must escape it by adding a \ in front of it.

Run the code below to see what it prints.

import re str1 = "me. you? us. them. when!" res1 = re.findall("\w+\.", str1) print(res1) res2 = re.findall("\w+\?", str1) print(res2) str3 = "a + b and c + d" res3 = re.findall("\w \+ \w", str3) print(res3)

11-9-19: How many items will be in the list that the following code prints?

import re
str = "302.33 64.52 204.24 532.2 1.23 323.320"
res = re.findall("\d{3}\.\d{2}",str)
print(res)

• 1
• It will match three digits followed by a period and then 2 digits
• 2
• It will match three digits followed by a period and then 2 digits
• 3
• It will match three digits followed by a period and then 2 digits
• 4
• It will match three digits followed by a period and then 2 digits

12.14.7. Greedy and Non-Greedy Matching

Matching is usually greedy.

Run the code below to see what it prints.

import re str1 = 'From: Using the : character' res1 = re.findall('F.+:', str1) print(res1) res2 = re.findall('F.+?:', str1) print(res2)

If you worked in a group, you can copy the answers from this page to the other group members. Select the group members below and click the button to share answers.

The Submit Group button will submit the answer for each each question on this page for each member of your group. It also logs you as the official group submitter.

You have attempted of activities on this page