MATLAB Programming Fundamentals - MathWorks

Lookahead Assertions in Regular Expressions

In this section...

“Lookahead Assertions” on page 2-57

“Overlapping Matches” on page 2-58

“Logical AND Conditions” on page 2-58

Lookahead Assertions

There are two types of lookaround assertions for regular expressions: lookahead and

lookbehind. In both cases, the assertion is a condition that must be satisfied to return a

match to the expression.

A lookahead assertion has the form (?=test) and can appear anywhere in a regular

expression. MATLAB looks ahead of the current location in the text for the test condition.

If MATLAB matches the test condition, it continues processing the rest of the expression

to find a match.

For example, look ahead in a character vector specifying a path to find the name of the

folder that contains a program file (in this case, fileread.m).

chr = which('fileread')

chr =

'matlabroot\toolbox\matlab\iofun\fileread.m'

regexp(chr,'\w+(?=\\\w+\.[mp])','match')

ans =

1×1 cell array

{'iofun'}

The match expression, \w+, searches for one or more alphanumeric or underscore

characters. Each time regexp finds a term that matches this condition, it looks ahead for

a backslash (specified with two backslashes, \\), followed by a file name (\w+) with an .m

or .p extension (\.[mp]). The regexp function returns the match that satisfies the

lookahead condition, which is the folder name iofun.

Lookahead Assertions in Regular Expressions