Exercism.io challenge - Bob

Solution to Bob challenge using regular expressions and the re-matches function.

Using re-matchers, if the string matches the pattern, then the string is returned. Otherwise nil is returned

The regular expressions cheatsheet from Mozilla Developer Network was very helpful in understanding regular expressions

Asking Bob a question?

The phrase passed to Bob is a question if the last alphanumeric character is a question mark. Using a simple regular expression we can check if the last character in the string a ?

#"\?" is a literal regular expression pattern that will match a single ? character

So the regular expression pattern will match a single ? character

(re-matches #"\?" "?")

With other characters present though the pattern doesn't match.

(re-matches #"\?" "Ready?")

To match ? with other characters,

. matches any single character except line terminators (new line, carriage return)

(re-matches #".\?" "R?")

.* matches any number of single characters one or more times,

(re-matches #".*\?" "?Ready")

\s matches a single whitespace character and \s* matches multiple whitespace characters

(re-matches #".*\?$" "Okay if like my  spacebar  quite a bit?")
  ;; => "Okay if like my  spacebar  quite a bit?"

$ is a boundary assertion so the pattern only matches the ? at the end of a string and not in the middle. However, this is not required as the re-matches uses groups and that manages the boundary assertion.

re-matches does not require the $ as there is an implicit boundary

(re-matches #".*\?" "Okay if like my ? spacebar  quite a bit")

Match if there is a single space or space type character after the ?

(re-matches #".*\?\s" "Okay if like my  spacebar  quite a bit? ")
  ;; => "Okay if like my  spacebar  quite a bit? "

Match if there are multiple space type characters after the ?

(re-matches #".*\?\s*" "Okay if like my  spacebar  quite a bit?   ")
  ;; => "Okay if like my  spacebar  quite a bit?   "

Dont match if a question mark character is not at the end of the string

(re-matches #".*\?" "Okay if like my ? spacebar  quite a bit")

Shouting a question at Bob

[^a-z] matches if there are no lower case alphabetic characters. The ^ at the start of the pattern negated the pattern.

* any number of the proceeding pattern

[A-Z]+ any number of upper case alphabetic characters

When a phrase has all uppercase characters then we have a match

(re-matches #"[^a-z]*[A-Z]+[^a-z]*" "HELLO")

If there are lower case characters, even if there are uppercase characters, the pattern does not match.

(re-matches #"[^a-z]*[A-Z]+[^a-z]*" "Hello")

If the characters are all uppercase then the pattern matches, even if there are other non-alphabetic characters

(re-matches #"[^a-z]*[A-Z]+[^a-z]*" "ABC 1 2 3")

Silence of the Bob

\s matches any single whitespace character, including space, tab, form feed, line feed, and other Unicode spaces.

(re-matches #"\s*" "  \t\t\t")

Solution using regular expressions

The re-matches expressions with regular expressions patterns can be put into a let expression. The names are bound to the re-matches expressions which evaluated to either true or false

The names from the let are used with a cond function as conditions, returning the relevant reply from Bob.

For the shouting question, the and is used to check if two names are both true.

(defn response-for
  (let [;; A ? at the end of the phrase, not counting whitespace
        question (re-matches #".*\?\s*" phrase)

        ;; No lower case characters, at least one upper case character
        yelling (re-matches #"[^a-z]*[A-Z]+[^a-z]*" phrase)

        ;; The entire string is whitespace
        silence (re-matches #"\s*" phrase)]

      silence                "Fine. Be that way!"
      (and question yelling) "Calm down, I know what I'm doing!"
      question               "Sure."
      yelling                "Whoa, chill out!"
      :whatever              "Whatever.")))

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