Regular Expressions
In this section...
“What Is a Regular Expression?” on page 2-51
“Steps for Building Expressions” on page 2-52
“Operators and Characters” on page 2-55
This topic describes what regular expressions are and how to use them to search text. Regular
expressions are flexible and powerful, though they use complex syntax. An alternative to regular
expressions is a pattern (since R2020b), which is simpler to define and results in code that is easier
to read. For more information, see “Build Pattern Expressions” on page 6-40.
What Is a Regular Expression?
A regular expression is a sequence of characters that defines a certain pattern. You normally use a
regular expression to search text for a group of words that matches the pattern, for example, while
parsing program input or while processing a block of text.
The character vector 'Joh?n\w*' is an example of a regular expression. It defines a pattern that
starts with the letters Jo, is optionally followed by the letter h (indicated by 'h?'), is then followed
by the letter n, and ends with any number of word characters, that is, characters that are alphabetic,
numeric, or underscore (indicated by '\w*'). This pattern matches any of the following:
Jon, John, Jonathan, Johnny
Regular expressions provide a unique way to search a volume of text for a particular subset of
characters within that text. Instead of looking for an exact character match as you would do with a
function like strfind, regular expressions give you the ability to look for a particular pattern of
characters.
For example, several ways of expressing a metric rate of speed are:
km/h
km/hr
km/hour
kilometers/hour
kilometers per hour
You could locate any of the above terms in your text by issuing five separate search commands:
strfind(text, 'km/h');
strfind(text, 'km/hour');
% etc.
To be more efficient, however, you can build a single phrase that applies to all of these search terms:
Regular Expressions
2-51