{'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 character
vector 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*', 'match')
i
p
p
ans =
1×1 cell array
{'mississippi'}
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 =
1×1 cell array
{'mississippi'}
But by inserting a dynamic script, you can see that this time, MATLAB has matched the text quite
differently. In this case, MATLAB uses the very first match it can find, and does not even consider the
rest of the text:
regexp('mississippi', '\w*?(\w)(?@disp($1))\1\w*', 'match')
m
i
s
ans =
1×1 cell array
{'mississippi'}
To demonstrate how versatile this type of dynamic expression can be, consider the next example that
progressively assembles a cell array as MATLAB iteratively parses the input text. The (?!) operator
found at the end of the expression is actually an empty lookahead operator, and forces a failure at
each iteration. This forced failure is necessary if you want to trace the steps that MATLAB is taking to
resolve the expression.
MATLAB makes a number of passes through the input text, each time trying another combination of
letters to see if a fit better than last match can be found. On any passes in which no matches are
Dynamic Regular Expressions
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