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The regular definition for recognizing identifiers in C programming language is given by
letter -> a|b|...z|A|B|...|Z|_ digit -> 0|1|...|9 identifier -> letter(letter|digit)* This definition will generate identifiers of the form
identifier: [_a-zA-Z][_a-zA-Z0-9]*
My question now is how do you limit the length of the identifier that can be generated to not more than 31 characters. What changes need to be made in the regular definition or how to write a regular expression to limit it to not more than the specified length. Could anyone please help. Thanks.
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identifier = letter (letter | digit)* real-numeral = digit digit* .
A regular expression is a sequence of characters that is used to search pattern. It is mainly used for pattern matching with strings, or string matching, etc. They are a generalized way to match patterns with sequences of characters. It is used in every programming language like C++, Java, and Python.
The backslash in combination with a literal character can create a regex token with a special meaning. E.g. \d is a shorthand that matches a single digit from 0 to 9. Escaping a single metacharacter with a backslash works in all regular expression flavors.
?= is a positive lookahead, a type of zero-width assertion. What it's saying is that the captured match must be followed by whatever is within the parentheses but that part isn't captured. Your example means the match needs to be followed by zero or more characters and then a digit (but again that part isn't captured).
The regular expression you are looking for is:
[_a-zA-Z][_a-zA-Z0-9]{0,30} It will match an underscore or letter following by X underscores, letters or numbers, where 0 <= X <= 30
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