MATLAB Programming

Regular Expressions
The following example parses a word for zero or more characters followed by
two identical characters followed again by zero or more characters:
regexp('mississippi', '\w*(\w)\1\w*', 'match')
ans =
'mississippi'
To track the exact steps that MATLAB takes in determining the match, the
example inserts a short script (?@disp($1)) in the expression to display the
characters that finally constitute the match. Because the example uses greedy
quantifiers, MATLAB attempts to match as much of the string as possible.
So, even though MATLAB finds a match toward the beginning of the string,
it continues to look for more matches until it arrives at the very end of the
string. From there, it backs up through the letters i then p and the next p,
stopping at that point because the match is finally satisfied:
regexp('mississippi', '\w*(\w)(?@disp($1))\1\w*');
i
p
p
Now try the same example again, this time making the first quantifier lazy
(*?). Again, MATLAB makes the same match:
regexp('mississippi', '\w*?(\w)\1\w*', 'match')
ans =
'mississippi'
But by inserting a dynamic script, you can see that this time, MATLAB has
matched the string quite differently. In this case, MATLAB uses the very first
match it can find, and does not even consider the rest of the string:
regexp('mississippi', '\w*?(\w)(?@disp($1))\1\w*';)
m
i
s
Todemonstratehowversatilethistypeof dynamic expression can be, consider
the next example that progressively assembles a cell array as MATLAB
iteratively parses the input string. The (?!) metacharacter found at the end
of the expression is actually an empty lookahead operator, and forces a failure
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