Why aren't Perl's patterns regular expressions?

Perl Questions and Answers

(Continued from previous question...)

Why aren't Perl's patterns regular expressions?

Because Perl patterns have backreferences.
A regular expression by definition must be able to determine the next state in the finite automaton without requiring any extra memory to keep around previous state. A pattern /([ab]+)c\1/ requires the state machine to remember old states, and thus disqualifies such patterns as being regular expressions in the classic sense of the term.

(Continued on next question...)

Other Interview Questions

• Why do you use Perl?
• How do I set environment variables in Perl programs?
• Which of these is a difference between C++ and Perl?
• How to open and read data files with Perl
• How do I do fill_in_the_blank for each file in a directory?
• How do I generate a list of all .html files in a directory?
• What is Perl one-liner?
• Assuming both a local($var) and a my($var) exist, what's the difference between ${var} and ${"var"}?
• What happens when you return a reference to a private variable?
• How to turn on Perl warnings? Why is that important?
• What are scalar data and scalar variables?
• Why should I use the -w argument with my Perl programs?
• Assuming $_ contains HTML, which of the following substitutions will remove all tags in it? 1.s/<.*>//g; 2.s/<.*?>//gs; 3.s/<\/?[A-Z]\w*(?:\s+[A-Z]\w*(?:\s*=\s*(?:(["']).*?\1|[\w-.]+))?)*\s*>//gsix;
• I want users send data by formmail but when they send nothing or call it from web site they will see error. codes in PHP like this: if (isset($HTTP_POST_VARS)){ .......... } else{ echo ("error lalalalal") } How it will look in perl?
• What is the output of the following Perl program? 1 $p1 = "prog1.java"; 2 $p1 =~ s/(.*)\.java/$1.cpp/; 3 print "$p1\n";
• Why aren't Perl's patterns regular expressions?
• What does Perl do if you try to exploit the execve(2) race involving setuid scripts?
• How do I do < fill-in-the-blank > for each element in a hash?
• How do I sort a hash by the hash key?
• How do you print out the next line from a filehandle with all its bytes reversed?
• How do I send e-mail from a Perl/CGI program on a Unix system?
• How to read from a pipeline with Perl
• Why is it hard to call this function: sub y { "because" }
• What does `$result = f() .. g()' really return?
• Why does Perl not have overloaded functions?
• What does read() return at end of file?
• What does `new $cur->{LINK}' do? (Assume the current package has no new() function of its own.)
• How do I sort a hash by the hash value?
• How to read file into hash array ?
• How do you find the length of an array?
• What value is returned by a lone `return;' statement?
• What's the difference between /^Foo/s and /^Foo/?
• Does Perl have reference type?
• How to dereference a reference?
• What does length(%HASH) produce if you have thirty-seven random keys in a newly created hash?
• If EXPR is an arbitrary expression, what is the difference between $Foo::{EXPR} and *{"Foo::".EXPR}?
• How do I do < fill-in-the-blank > for each element in an array?
• How do I replace every <TAB> character in a file with a comma?
• What is the easiest way to download the contents of a URL with Perl?
• How to concatenate strings with Perl?
• How do I read command-line arguments with Perl?
• When would `local $_' in a function ruin your day?
• What happens to objects lost in "unreachable" memory..... ?
• Assume that $ref refers to a scalar, an array, a hash or to some nested data structure. Explain the following statements:
• How do you match one letter in the current locale?
• How do I print the entire contents of an array with Perl?
• Perl uses single or double quotes to surround a zero or more characters. Are the single(' ') or double quotes (" ") identical?
• How many ways can we express string in Perl?
• How do you give functions private variables that retain their values between calls?