Instead of talking about a specific regex, let\'s consider the language of regul
ID: 3111174 • Letter: I
Question
Instead of talking about a specific regex, let's consider the language of regular expressions (as strings) over the alphabet {0, 1, epsilon, [, ], 0, U, o, *}^1 [epsilon represents the empty string as a character, o represents concatenation as a character, U represents union as a character, * represents Kleene star as a character, and [, ] represent parentheses/brackets as characters). R = (0 Union 1 Union epsilon Union 0) because brackets need to be matched up. Similar reasoning goes for U, o, *. Create a regex S that recognizes exactly the language of such regexes. ^3 Explain, in English, why your regex is correct For partial credit, you can instead handle the case of no brackets at all as is described in Part 3 below. (If you do so for Part 1, you won't lose any points in Part 2 if applied correctly.)Explanation / Answer
If We have a file in which all lines are sorted (alphabetically or otherwise), you can easily delete (subsequent)
duplicate lines. Simply open the file in your favorite text editor, and do a search-and-replace searching for
«^(.*)( ? )+$» » matches a single-line string that does not allow the quote character to appear inside the string. Using the negated character class ismore efficient than using a lazy dot. «
"[^"]*» allows the
string to span across multiple lines.
«"[^"\ ]*(?:\.[^"\ ]*)*"» matches a single-line string in which the quote character can
appear if it is escaped by a backslash. Though this regular expression may seem more complicated than it
needs to be, it is much faster than simpler solutions which can cause a whole lot of backtracking in case a
double quote appears somewhere all by itself rather than part of a string. «"[^"\]*(?:\.[^"\]*)*"
» allows the string to span multiple lines.
We can adapt the above regexes to match any sequence delimited by two (possibly different) characters. If
we use “b” for the starting character, “e” and the end, and “x” as the escape character, the version without
escape becomes «b[^e ]*e», and the version with escape becomes
«b[^ex ]*(?:x.[^ex ]*)*e».