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I know this question seems stupid, but it isn't. I mean what is it exactly. I have a fair understanding of the parsing problem. I know BNF/EBNF, I've written grammar to parse simple context-free languages in one of my college courses. I just never met regular expressions before! The only thing that I remember about it is that context-free grammar can do all what regular expression can do.
Also, is it useful for a usual coding to parse strings? A simple example would be helpful.
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A Regular Expression (or Regex) is a pattern (or filter) that describes a set of strings that matches the pattern. In other words, a regex accepts a certain set of strings and rejects the rest.
Solution: As we know, any number of a's means a* any number of b's means b*, any number of c's means c*. Since as given in problem statement, b's appear after a's and c's appear after b's. So the regular expression could be: R = a* b* c*
Regular expressions are particularly useful for defining filters. Regular expressions contain a series of characters that define a pattern of text to be matched—to make a filter more specialized, or general. For example, the regular expression ^AL[.]* searches for all items beginning with AL.
The term regular expression comes from mathematics and computer science theory, where it reflects a trait of mathematical expressions called regularity. The text patterns used by the earliest grep tools were regular expressions in the mathematical sense.
Regular expressions first came around in mathematics and automata theory. A regular expression is simply something which defines a regular language. Without going too much into what "regular" means, think of a language as this way:
So you could have a string (which is, remember, just a concatenation of symbols) which is not part of a given language. Or it could be in the language.
So lets say you have an alphabet made of 2 symbols: "0" and "1". And lets say you want to create a language using the symbols in that alphabet. You could create the following rule: "In order for a string to be in my language, it must have only 0's and 1's in it."
So these strings are in your language:
These would not be in your language:
That's a pretty simple language. How about this: "In my language, each string [analogous to a valid 'word' in English] must being with a 0, and then can be followed by any number of 0's or 1's"
These are in the language:
These are not:
Well rather than defining the language using words - and these languages might get very complex ("1 followed by 2 0's followed by any combination of 1's and 0's ending with a 1"), we came up with this syntax called "regular expressions" to define the language.
The first language would have been:
(0|1)*
(0 or 1, repeated infinitely)
The next: 0(0|1)*
(0, followed by any number of 0's and 1's).
So lets think of programming now. When you create a regex, you are saying "Look at this text. Return to me strings which match this pattern." Which is really saying "I have defined a language. Return to me all strings within this document which are in my language."
So when you create a "regex", you are actually defining a regular language, which is a mathematical concept. (In actuality, perl-like regex define "nonregular" languages, but that is a separate issue.)
By learning the syntax of regex, you are learning the ins and outs of how to create a language, so that later you can see if a given string is "in" the language. Thus, commonly, people say that regex are for pattern matching - which is basically what you are doing when you look at a pattern, and see if it "matches" the rules for your language.
(this was long. does it answer your question at all?)
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