Lookahead Assertions in Regular Expressions
In this section...
“Lookahead Assertions” on page 2-63
“Overlapping Matches” on page 2-63
“Logical AND Conditions” on page 2-64
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.
Overlapping Matches
Lookahead assertions do not consume any characters in the text. As a result, you can use them to find
overlapping character sequences.
For example, use lookahead to find every sequence of six nonwhitespace characters in a character
vector by matching initial characters that precede five additional characters:
chr = 'Locate several 6-char. phrases';
startIndex = regexpi(chr,'\S(?=\S{5})')
startIndex =
1
8
9
16
17
24
25
Lookahead Assertions in Regular Expressions
2-63