问题
I know a little about what is a turing-machine and a turing-complete language, but to understand better, could someone give examples of languages that are not Turing complete? (maybe even machines that are not Turing, as well?)
回答1:
Regular expressions, in the formal definition, consisting only of:
- concatenation ( ab )
- unbounded repetition ( a* )
- alternation ( a|b )
- grouping ( (ab)|(cd) )
can only recognise regular languages. A Turing-complete programming language can recognise recursively-enumerable languages.
An example is that regular expressions cannot tell you if a string consists of matched pairs of parentheses: eg ()(()) is accepted while ()((())() is rejected, while Turing-complete programming languages can.
(Note that regexes in modern programming languages are more powerful than the formal academic definition of regular expressions. Some may even be Turing complete.)
回答2:
Regular languages - those that can be described as regular expressions - are not Turing complete.
Markup languages (used for describing data, not computation) like XML and JSON are not Turing complete.
来源：https://stackoverflow.com/questions/3600316/looking-for-languages-that-are-not-turing-complete