Skip to content

String matching

On rare occasions string matching is useful.

A much more useful feature is the Damerau-Levenshtein distance.

Since student's expressions are, by default, not permitted to contain spaces you will probably need to make use of the string input type.


This test performs a string match ignoring leading and trailing white space which are stripped from all answers, using PHP's trim() function.


This test removes spaces, newlines and tabs and then performs a string match.


This test uses Maxima's regex_match function.

  • Both arguments to the test must be Maxima strings. If you have a general expression, turn it into a string in the feedback variables with Maxima's string function.
  • The first argument should be the string, and the second argument should be the pattern to match.
  • It yields true if the pattern is matched anywhere within the student answer and false otherwise. Testing for full equality of the answer string can be achieved via regex anchoring by use of ^ or $.
  • Don't forget to escape within the pattern strings as needed. Note that there is a function string_to_regex() that will handle escaping of characters that would otherwise have meaning in the pattern. Also remember that you need to escape the backslashes like normal in Maxima-strings. That is to say, if you want to use \s in a pattern you need to double up the backslashes. For example "(Alice|Bob)\\s+went\\s+to\\s+the\\s+(bank|market)".
  • One can read more about the patterns possible from here. Case-insensitivity may be something worth noting there.

For example, write a STACK question with the following question variables.

s2:"Alice went to the market";
s3:"Bob       went to the    bank";


  1. Use the string input (ans1) with teacher's answer s2.
  2. In the PRT use the node ATSRegExp(ans1, s1). This will use the pattern in the string s1 against the student's answer.

This will match both strings s2 and s3, and many others.


STACK also provides a helper function regex_match_exactp(regex, str) to check if the string equals the pattern matched by the regular expression.

Regex           String      Result
(aaa)*(b|d)c    aaaaaabc    true
(aaa)*(b|d)c    dc          true
(aaa)*(b|d)c    aaaaaaabc   false
(aaa)*(b|d)c    cca         false

Currently this is not provided as a separate answer test so you will need to use this predicate in the question variables and check the result against the expected value, or supply the predicate as an argument to an answer test.

For example, using the question variables above you can define the following in the feedback variables.

sa:regex_match_exactp(s1, ans1);

Then you can use the AlgEquiv answer test to check the result is true/false, e.g. ATAlgEquiv(sa, true).

(RegExp) NOTE: this test was removed in STACK version 4.3.