Regular Expressions

Last updated on 2024-09-30 | Edit this page

Estimated time: 35 minutes

Overview

Questions

  • How can you imagine using regular expressions in your work?

Objectives

  • Identify potential use cases for regular expressions
  • Recognize common regex metacharacters
  • Use regular expressions in searches

Regular expressions

Regular expressions are a concept and an implementation used in many different programming environments for sophisticated pattern matching. They are an incredibly powerful tool that can amplify your capacity to find, manage, and transform data and files.

A regular expression, often abbreviated to regex, is a method of using a sequence of characters to define a search to match strings, i.e. “find and replace”-like operations. In computation, a ‘string’ is a contiguous sequence of symbols or values. For example, a word, a date, a set of numbers (e.g., a phone number), or an alphanumeric value (e.g., an identifier). A string could be any length, ranging from empty (zero characters) to one that spans many lines of text (including line break characters). The terms ‘string’ and ‘line’ are sometimes used interchangeably, even when they are not strictly the same thing.

In library searches, we are most familiar with a small part of regular expressions known as the “wild card character,” but there are many more features to the complete regular expressions syntax. Regular expressions will let you:

  • Match on types of characters (e.g. ‘upper case letters’, ‘digits’, ‘spaces’, etc.).
  • Match patterns that repeat any number of times.
  • Capture the parts of the original string that match your pattern.

Regex can also be useful for daily work. For example, say your organization wants to change the way they display telephone numbers on their website by removing the parentheses around the area code. Rather than search for each specific phone number (that could take forever and be prone to error) or searching for every open parenthesis character (could also take forever and return many false-positives), you could search for the pattern of a phone number. Regular expressions rely on the use of literal characters and metacharacters. A metacharacter is any American Standard Code for Information Interchange (ASCII) character that has a special meaning. By using metacharacters and possibly literal characters, you can construct a regex for finding strings or files that match a pattern rather than a specific string.

Since regular expressions defines some ASCII characters as “metacharacters” that have more than their literal meaning, it is also important to be able to “escape” these metacharacters to use them for their normal, literal meaning. For example, the period . means “match any character”, but if you want to match a period then you will need to use a \ in front of it to signal to the regular expression processor that you want to use the period as a plain old period and not a metacharacter. That notation is called “escaping” the special character. The concept of “escaping” special characters is shared across a variety of computational settings, including markdown and Hypertext Markup Language (HTML).

Regex Syntax and interoperability

Most regular expression implementations employ similar syntaxes and metacharacters (generally influenced by the regex syntax of a programming language called Perl), and they behave similarly for most pattern-matching in this lesson. But there are differences, often subtle, in each, so it’s always a good practice to read the application or language’s documentation whenever available, especially when you start using more advanced regex features. Some programs, notably many UNIX command line programs (for more on UNIX see our ‘Shell Lesson’), use an older regex standard (called ‘POSIX regular expressions’) which is less feature-rich and uses different metacharacters than Perl-influenced implementations. For the purposes of our lesson, you do not need to worry too much about all this, but if you want to follow up on this see this detailed syntax comparison.

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 because it locates all matches for the pattern in the file, not just for that word, it would also match reorganise, reorganize, organises, organizes, organised, organized, etc.

Learning common regex metacharacters

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.
  • [A-Za-z0-9] matches any upper or lower case letter or any digit.

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.
  • \ 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.
  • ^ is an “anchor” which 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.
  • $ is an “anchor” which 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 asserts that the pattern must match at a word boundary. Putting this either side of a word stops the regular expression matching longer variants of words. So:
    • the regular expression mark will match not only mark but also find marking, market, unremarkable, and so on.
    • the regular expression \bword will match word, wordless, and wordlessly.
    • the regular expression comb\b will match comb and honeycomb but not combine.
    • the regular expression \brespect\b will match respect but not respectable or disrespectful.

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

Using special characters in regular expression matches

What will the regular expression ^[Oo]rgani.e\b match?

organise
organize
Organise
Organize
organife
Organike

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. See solution visualised on Regexper.com

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]).

So, what are these going to match?

^[Oo]rgani.e\w*

What will the regular expression ^[Oo]rgani.e\w* match?

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+$

What will the regular expression [Oo]rgani.e\w+$ match?

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

What will the regular expression ^[Oo]rgani.e\w?\b match?

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?$

What will the regular expression ^[Oo]rgani.e\w?$ match?

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

What will the regular expression \b[Oo]rgani.e\w{2}\b match?

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

What will the regular expression \b[Oo]rgani.e\b|\b[Oo]rgani.e\w{1}\b match?

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. 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. There are many other contexts in which regex is useful when using a computer to search through a document, spreadsheet, or file structure. Some real-world use cases for regex were included on a ACRL Tech Connect blog post now archived at Library Hat .

To embed this knowledge we will not - however - be using computers. Instead we’ll use pen and paper for now.

Exercise

Work in teams of four to six on the exercises below. When you think you have the right answer, check it against the solution.

When you finish, 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, regex pal or regexper.com: the first three help you see what text your regular expression will match, the latter visualises the workflow of a regular expression.

Using square brackets

What will the regular expression Fr[ea]nc[eh] match?

French
France
Frence
Franch

Note that the way this regular expression is constructed, it will match misspellings such as Franch and Frence. Lacking an “anchor” such as ^ or \b, this will also find strings where there are characters to either side of the regular expression, such as in French, France's, French-fried.

Using dollar signs

What will the regular expression Fr[ea]nc[eh]$ match?

French
France
Frence
Franch

This will match the pattern only when it appears at the end of a line. It will also find strings with other characters coming before the pattern, for example, in French or faux-French.

Introducing options

What would match the strings French and France that appear at the beginning of a line?

^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)?

\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. To use these flags, include / delimiters before and after the expression then letters after to raise each flag (where i is ‘ignore case’): 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?

\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 four letters preceded by at least one zero?

0+[A-Za-z]{4}\b

Matching digits

How do you match any four-digit string anywhere?

\d{4}

Note: this will also match four-digit strings within longer strings of numbers and letters.

Matching dates

How would you match the date format dd-MM-yyyy?

\b\d{2}-\d{2}-\d{4}\b

Depending on your data, you may choose 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?

\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 choose 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?

.* ?: .*, \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

  • Regular expressions are a language for pattern matching.