Matching & Extracting Strings
Last updated on 2024-04-28 | Edit this page
Overview
Questions
- How can you use regular expressions to match and extract strings?
Objectives
- Use regular expressions to match words, email addresses, and phone numbers.
- Use regular expressions to extract substrings from strings (e.g. addresses).
Exercise Using Regex101.com
For this exercise, open a browser and go to https://regex101.com. Regex101.com is a free regular expression debugger with real time explanation, error detection, and highlighting.
Open the swcCoC.md file, copy the text, and paste that into the test string box.
For a quick test to see if it is working, type the string
community
into the regular expression box.
If you look in the box on the right of the screen, you see that the expression matches six instances of the string ‘community’ (the instances are also highlighted within the text).
Taking spaces into consideration
Type community
. You get three matches. Why not six?
The string ‘community-led’ matches the first search, but drops out of
this result because the space does not match the character
-
.
Taking any character into consideration
If you want to match ‘community-led’ by adding other regex characters
to the expression community
, what would they be?
For instance, \S+\b
. This would match one or more
non-space characters followed by a word boundary.
Exploring effect of expressions matching different words
Change the expression to communi
and you get 15 full
matches of several words. Why?
Because the string ‘communi’ is present in all of those words,
including communi
cation and communi
ty. Because
the expression does not have a word boundary, this expression would also
match incommuni
cado, were it present in this text. If you
want to test this, type incommunicado
into the text
somewhere and see if it is found.
Taking capitalization into consideration
Type the expression [Cc]ommuni
. You get 16 matches.
Why?
The pattern communi
with either a capital C
or lowercase c
is present in the text 16 times.
Regex characters that indicate location
Type the expression ^[Cc]ommuni
. You get no matches.
Why?
There is no matching string present at the start of a line. Look at
the text and replace the string after the ^
with something
that matches a word at the start of a line. Does it find a match?
Finding plurals
Find all of the words starting with Comm or comm that are plural.
[Cc]omm\w+s\b
[Cc]
finds capital and lowercase c
omm
is straightforward character matches
\w+
matches the preceding element (a word character) one
or more times
s
is a straightforward character match
\b
ensures the ‘s’ is located at the end of the
word.
Exercise finding email addresses using regex101.com
For this exercise, open a browser and go to https://regex101.com.
Open the swcCoC.md file, copy it, and paste it into the test string box.
Start with what you know
What character do you know is held in common with all email addresses?
The ‘@’ character.
Add to what you know
The string before the “@” could contain any kind of word character, special character or digit in any combination and length. How would you express this in regex? Hint: often addresses will have a dash (-) or dot (.) in them, and neither of these are included in the word character expression (\w). How do you capture this in the expression?
[\w.-]+@
\w
matches any word character (including digits and
underscore)
.
matches a literal period (when used in between square
brackets, .
does not mean “any character”, it literally
means “.”)
-
matches a dash
[]
the brackets enclose the Boolean string that ‘OR’ the
word characters, dot, and dash.
+
matches any word character OR digit OR character OR
-
repeated 1 or more times
Finish the expression
The string after the “@” could contain any kind of word character,
special character or digit in any combination and length as well as the
dash. In addition, we know that it will have some characters after a
period (.
). Most common domain names have two or three
characters, but many more are now possible (access the
latest list). What expression would capture this? Hint: the
.
is also a metacharacter, so you will have to use the
escape \
to express a literal period. Note: for the string
after the period, we did not try to match a -
character,
since those rarely appear in the characters after the period at the end
of an email address.
[\w.-]+\.\w{2,3} OR [\w.-]+\.\w+
See the previous exercise for the explanation of the expression up to
the +
\.
matches the literal period (‘.’) not the regex
expression .
\w
matches any word (including digits and
underscore)
+
matches any word character OR digit OR character OR
-
repeated 1 or more times.
{2,3}
limits the number of word characters and/or digits
to a two or three-character string.
[]
the brackets enclose the Boolean string that ‘OR’ the
digits, word characters, characters and dash.
+
matches any word character OR digit OR character OR
-
repeated 1 or more times
Exercise finding phone numbers, Using regex101.com
Does this Code of Conduct contain a phone number?
What to consider:
- It may or may not have a country code, perhaps starting with a “+”.
- It will have an area code, potentially enclosed in parentheses.
- It may have the sections all separated with a “-”.
Start with what you know: find strings of digits
Start with what we know, which is the most basic format of a phone number: three digits, a dash, and four digits. How would we write a regex expression that matches this?
\d{3}-\d{4}
\d
matches digits
{3}
matches 3 digits
-
matches the character ‘-’
\d
matches any digit
{4}
matches 4 digits.
This expression should find three matches in the document.
Match a string that includes an area code with a dash
Start with what we know, which is the most basic format of a phone number: three digits, a dash, and four digits. How would we expand the expression to include an area code (three digits and a dash)?
\d{3}-\d{3}-\d{4}
\d
matches digits
{3}
matches 3 digits
-
matches the character ‘-’
\d
matches any digit
{4}
matches 4 digits.
This expression should find one match in the document
Match a string that includes an area code within parenthesis separated from the rest of the phone number with a space or without a space
Start with what we know, which is the most basic format of a phone number: three digits, a dash, and four digits. How would we expand the expression to include a phone number with an area code in parenthesis, separated from the phone number, with or without a space.
\(\d{3}\) ?\d{3}-\d{4}
\(
escape character with the parenthesis as
straightforward character match
\d
matches digits
{3}
matches 3 digits
\)
escape character with the parenthesis as a
straightforward character match
?
matches zero or one spaces
See the previous exercise for the explanation of the rest of the expression.
This expression should find two matches in the document.
Match a phone number containing a country code.
Country codes are preceded by a “+” and can have up to three digits. We also have to consider that there may or may not be a space between the country code and anything appearing next.
\+\d{1,3} ?\(\d{3}\)\s?\d{3}-\d{4}
\+
escape character with the plus sign as
straightforward character match
\d
matches digits
{1,3}
matches 1 to 3 digits
?
matches zero or one spaces
See the previous exercise for the explanation of the rest of the expression.
This expression should find one match in the document.
Using regular expressions when working with files and directories
One of the reasons we stress the value of consistent and predictable directory and filenaming conventions is that working in this way enables you to use the computer to select files based on the characteristics of their file names. For example, if you have a bunch of files where the first four digits are the year and you only want to do something with files from ‘2017’, then you can. Or if you have ‘journal’ somewhere in a filename when you have data about journals, you can use the computer to select just those files. Equally, using plain text formats means that you can go further and select files or elements of files based on characteristics of the data within those files. See Workshop Overview: File Naming & Formatting for further background.
Extracting a substring in Google Sheets using regex
Extracting a substring in Google Sheets using regex
- Export and unzip the 2017 Public Library Survey (originally from the IMLS data site) as a CSV file.
- Upload the CSV file to Google Sheets and open as a Google Sheet if it does not do this by default.
- Look in the
ADDRESS
column and notice that the values contain the latitude and longitude in parenthesis after the library address. - Construct a regular expression to match and extract the latitude and
longitude into a new column named ‘latlong’. HINT: Look up the function
REGEXEXTRACT
in Google Sheets. That function expects the first argument to be a string (a cell inADDRESS
column) and a quoted regular expression in the second.
This is one way to solve this challenge. You might have found others. Inside the cell you can use the below to extract the latitude and longitude into a single cell. You can then copy the formula down to the end of the column.
=REGEXEXTRACT(G2,"-?\d+\.\d+, -?\d+\.\d+")
Latitude and longitude are in decimal degree format and can be
positive or negative, so we start with an optional dash for negative
values then use \d+
for a one or more digit match followed
by a period \.
. Note we had to escape the period using
\
. After the period we look for one or more digits
\d+
again followed by a literal comma ,
. We
then have a literal space match followed by an optional dash
-
(there are few 0.0
latitude/longitudes that
are probably errors, but we’d want to retain so we can deal with them).
We then repeat our \d+\.\d+
we used for the latitude
match.
Key Points
- Regular expressions are useful for searching and cleaning data.
- Test regular expressions interactively with regex101.com or RegExr.com, and visualize them with regexper.com.
- Test yourself with RegexCrossword.com or via the quiz and exercises in this lesson.