Question

For each of the following Perl regular expressions, give a complete and precise description of the function of the regular expression, plus a related example string:

(a) /"([^"]*)"/

(b) /[-+]?\d+(\.\d*)?F\b/

(c) /(\D{2,}).*\[\1\]/

(d) /((.*?)\d)\s\2/

(e) /^[0-9]+\/\d+([+\-*\/]\=|([+]{2}|[-]{2}));$/

Answer 1

Q1)         The regular expression /"([^"]*)"/

"([^"]*)"

" "           Any text within double inverted commas.

Lets evaluate inside the brackets ( )

^"                     Anything except " (double inverted comma)

*                      It is a quantifier. Repetition of the preceding value 0 or any number of times. ie Anything except " (double inverted comma)

Example :

"This is an example"

Q2)         The regular expression /[-+]?\d+(\.\d*)?F\b/

[-+]?\d+(\.\d*)?F\b

[-+]                  Either a minus or plus sign.

[-+]?                ? means matches between zero and one times. So there can be minus, plus or no sign.

\d                     matches a digit (equal to [0-9]).

\d+                   digit repeated 1 or unlimited times.

\.                      matches character . (dot)        

\d*                   digit repeated zero or many times.

(\.\d*)?             ? means a decimal value related 0 or 1 time.

F                      Character F literally   

\b                     Matches word boundary. it means it has to end with F

This expression represent a negative or positive Fahrenheit value

Example :

-3.54F or +25.66F

Q3)         The regular expression /(\D{2,}).*\[\1\]/

(\D{2,}).*\[\1\]

\D                    Anything but not digit 0-9.

\D{2,}             Anything but not digit 0-9 matching 2 or more times ie two or more anything but not digits. Can be 2 characters like KR

.*                     matches any character but not end of line \n ie it shouldn't stop with KR alone

\[ \]                  means square brackets [ ]

\1                     matches the same text as most recently matched by (\D{2,}) ie 1st two characters, in our example KR

Example :

GABV[GA]                           : True

GABV dd[GA]                      : True

GBV[GA]                               : False

Q4)         The regular expression /((.*?)\d)\s\2/

(.*?)\d)\s\2

(.*?)                 matches any character (except for line terminators) zero or unlimited times.

\d                     matches a digit (equal to [0-9]).

\s                      matches any whitespace character like a tab or enter.

\2                     ((.*?)\d) is call the 1st group. and (.*?) is the 2nd group. \2 means match the 2nd group (.*?) ie characters 0 or more times

Any thing text or no text followed by a digit then a white space followed by text or no text.

Example :

I am 6 years old