Regular expressions exercises

Overview

Teaching: 45 min
Exercises: 17 min

Questions

Exercises to solve together

Objectives

Understand how to modify a regular expression to select the desired kind of text.

Summary

Square brackets can be used to define a list or range of characters to be found. So:

[ABC] matches A or B or C
[A-Z] matches any upper case letter
[A-Za-z] matches any upper or lower case letter (note: this is case-sensitive)
[A-Za-z0-9] matches any upper or lower case letter or any digit (note: this is case-sensitive)

Then there are:

. matches any character
\d matches any single digit
\w matches any part of word character (equivalent to [A-Za-z0-9])
\s matches any space, tab, or newline
\ NB: this is also used to escape the following character when that character is a special character. So, for example, a regular expression that found .com would be \.com because . is a special character that matches any character.
^ asserts the position at the start of the line. So what you put after the caret will only match if they are the first characters of a line. The caret is also known as a circumflex.
$ asserts the position at the end of the line. So what you put before it will only match if they are the last characters of a line.
\b adds a word boundary. Putting this either side of a word stops the regular expression matching longer variants of words. So:
- the regular expression foobar will match foobar and find 666foobar, foobar777, 8thfoobar8th et cetera
- the regular expression \bfoobar will match foobar and find foobar777
- the regular expression foobar\b will match foobar and find 666foobar
- the regular expression \bfoobar\b will find foobar

Other useful special characters are:

* matches the preceding element zero or more times. For example, ab*c matches “ac”, “abc”, “abbbc”, etc.
+ matches the preceding element one or more times. For example, ab+c matches “abc”, “abbbc” but not “ac”.
? matches when the preceding character appears zero or one time.
{VALUE} matches the preceding character the number of times defined by VALUE; ranges, say, 1-6, can be specified with the syntax {VALUE,VALUE}, e.g. \d{1,9} will match any number between one and nine digits in length.
| means or.
/i renders an expression case-insensitive (equivalent to [A-Za-z])

Exercises

A very simple use of a regular expression would be to locate the same word spelled two different ways. For example the regular expression organi[sz]e matches both “organise” and “organize”.

But it would also match reorganise, reorganize, organises, organizes, organised, organized, et cetera, because we have not specified the beginning or end of our string. So we need to use special syntax to help us be more precise.

We will continue using regex101.com/ to test our solutions. Copy and paste the data in regex_exercises_data.txt under “test string”. Select the flag ‘m’ (multiline) and the flag ‘g’ (global) so that you will get more than one match in one line, and matches in different lines and so that ^ and $ will match the start/end of the line.

So, what is ^[Oo]rgani.e\b going to match?

Using special characters in regular expression matches

Can you guess what the regular expression ^[Oo]rgani.e\b will match?
Solution
organise
organize
Organise
Organize
organife
Organike
Organi1e
Or, any other string that starts a line, begins with a letter o in lower or capital case, proceeds with rgani, has any character in the 7th position, and ends with the letter e.

So, what are these going to match?

^[Oo]rgani.e\w*

Can you guess what the regular expression ^[Oo]rgani.e\w* will match?
Solution
organise
Organize
organifer
Organi2ed111
Or, any other string that starts a line, begins with a letter o in lower or capital case, proceeds with rgani, has any character in the 7th position, follows with letter e and zero or more characters from the range [A-Za-z0-9].

[Oo]rgani.e\w+$

Can you guess what the regular expression [Oo]rgani.e\w+$ will match?
Solution
organiser
Organized
organifer
Organi2ed111
Or, any other string that ends a line, begins with a letter o in lower or capital case, proceeds with rgani, has any character in the 7th position, follows with letter e and at least one or more characters from the range [A-Za-z0-9].

^[Oo]rgani.e\w?\b

Can you guess what the regular expression ^[Oo]rgani.e\w?\b will match?
Solution
organise
Organized
organifer
Organi2ek
Or, any other string that starts a line, begins with a letter o in lower or capital case, proceeds with rgani, has any character in the 7th position, follows with letter e, and ends with zero or one characters from the range [A-Za-z0-9].

^[Oo]rgani.e\w?$

Can you guess what the regular expression ^[Oo]rgani.e\w?$ will match?
Solution
organise
Organized
organifer
Organi2ek
Or, any other string that starts and ends a line, begins with a letter o in lower or capital case, proceeds with rgani, has any character in the 7th position, follows with letter e and zero or one characters from the range [A-Za-z0-9].

\b[Oo]rgani.e\w{2}\b

Can you guess what the regular expression \b[Oo]rgani.e\w{2}\b will match?
Solution
organisers
Organizers
organifers
Organi2ek1
Or, any other string that begins with a letter o in lower or capital case after a word boundary, proceeds with rgani, has any character in the 7th position, follows with letter e, and ends with two characters from the range [A-Za-z0-9].

\b[Oo]rgani.e\b|\b[Oo]rgani.e\w{1}\b

Can you guess what the regular expression \b[Oo]rgani.e\b|\b[Oo]rgani.e\w{1}\b will match?
Solution
organise
Organi1e
Organizer
organifed
Or, any other string that begins with a letter o in lower or capital case after a word boundary, proceeds with rgani, has any character in the 7th position, and end with letter e, or any other string that begins with a letter o in lower or capital case after a word boundary, proceeds with rgani, has any character in the 7th position, follows with letter e, and ends with a single character from the range [A-Za-z0-9].

This logic is useful when you have lots of files in a directory, when those files have logical file names, and when you want to isolate a selection of files. Or it can be used for looking at cells in spreadsheets for certain values, or for extracting some data from a column of a spreadsheet to make new columns. I could go on. The point is, it is useful in many contexts. To embed this knowledge we won’t - however - be using computers. Instead we’ll use pen and paper. Work in teams of 4-6 on the exercises below. When you think you have the right answer, check it against the solution. When you finish, I’d like you to split your team into two groups and write each other some tests. These should include a) strings you want the other team to write regex for and b) regular expressions you want the other team to work out what they would match. Then test each other on the answers. If you want to check your logic, use regex101, myregexp, or regex pal regexper.com: the first three help you see what text your regular expression will match, the latter visualises the workflow of a regular expression.

Exercise

Pair up with the person next to you to work through the following problems.

Using square brackets

Can you guess what the regular expression Fr[ea]nc[eh] will match?
Solution
French
France
Frence
Franch
This will also find words where there are characters either side of the solutions above, such as Francer, foobarFrench, and Franch911.

Using dollar signs

Can you guess what the regular expression Fr[ea]nc[eh]$ will match?
Solution
French
France
Frence
Franch
This will also find strings at the end of a line. It will find words where there were characters before these, for example foobarFrench.

Introducing options

What would match the strings French and France only that appear at the beginning of a line?
Solution
^France|^French
This will also find words where there were characters after French such as Frenchness.

Case insensitivity

How do you match the whole words colour and color (case insensitive)?
Solutions
\b[Cc]olou?r\b|\bCOLOU?R\b
/colou?r/i
In real life, you should only come across the case insensitive variations colour, color, Colour, Color, COLOUR, and COLOR (rather than, say, coLour). So based on what we know, the logical regular expression is \b[Cc]olou?r\b|\bCOLOU?R\b. An alternative more elegant option we’ve not discussed is to take advantage of the / delimiters and add an ‘ignore case’ flag: so /colou?r/i will match all case insensitive variants of colour and color.

Word boundaries

How would you find the whole word headrest and or head rest but not head rest (that is, with two spaces between head and rest?
Solution
\bhead ?rest\b
Note that although \bhead\s?rest\b does work, it will also match zero or one tabs or newline characters between head and rest. So again, although in most real world cases it will be fine, it isn’t strictly correct.

Matching non-linguistic patterns

How would you find a string that ends with 4 letters preceded by at least one zero?
Solution
0+[A-Za-z]{4}\b

Matching digits

How do you match any 4-digit string anywhere?
Solution
\d{4}
Note: this will also match 4 digit strings within longer strings of numbers and letters.

Matching dates

How would you match the date format dd-MM-yyyy?
Solution
\b\d{2}-\d{2}-\d{4}\b
Depending on your data, you may chose to remove the word bounding.

Matching multiple date formats

How would you match the date format dd-MM-yyyy or dd-MM-yy at the end of a line only?
Solution
\d{2}-\d{2}-\d{2,4}$
Note this will also find strings such as 31-01-198 at the end of a line, so you may wish to check your data and revise the expression to exclude false positives. Depending on your data, you may chose to add word bounding at the start of the expression.

Matching publication formats

How would you match publication formats such as British Library : London, 2015 and Manchester University Press: Manchester, 1999?
Solution
.* ?: .*, \d{4}
Without word boundaries you will find that this matches any text you put before British or Manchester. Nevertheless, the regular expression does a good job on the first look up and may be need to be refined on a second, depending on your data.

Key Points

Select upper case or lower case; match any character; assert the position at the start and end of line; add a word boundary

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Data Intro for Librarians

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Regular expressions exercises

Overview

Summary

Exercises

Using special characters in regular expression matches

Solution

^[Oo]rgani.e\w*

Solution

[Oo]rgani.e\w+$

Solution

^[Oo]rgani.e\w?\b

Solution

^[Oo]rgani.e\w?$

Solution

\b[Oo]rgani.e\w{2}\b

Solution

\b[Oo]rgani.e\b|\b[Oo]rgani.e\w{1}\b

Solution

Exercise

Using square brackets

Solution

Using dollar signs

Solution

Introducing options

Solution

Case insensitivity

Solutions

Word boundaries

Solution

Matching non-linguistic patterns

Solution

Matching digits

Solution

Matching dates

Solution

Matching multiple date formats

Solution

Matching publication formats

Solution

Key Points

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