PerlRegularExpressionsbyExample

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Introduction

Regular expressions are very powerful tools for matching, searching, and replacing text. Unfortunately, they are also very obtuse. It does not help that most explanations of regular expressions start from the specification, which is like learning to love Friends reruns by reading a VCR manual. This page provides some simple examples for reference.

You should know a little programming and how to run basic perl scripts before reading this article.

Section 1: Basic matching and substitution

Declare a local variable called $mystring.

my $mystring;

Assign a value (string literal) to the variable.

$mystring = "Hello world!";

Does the string contains the word "World"?

if($mystring =~ m/World/) { print "Yes"; }

No, it doesn't. The binding operator =~ with the match operator m// does a pattern search on $mystringand returns true if the pattern is found. The pattern is whatever is between the m/ and the trailing /. (Note, there is no such thing as a ~= operator, and using it will give a compile error.)

Does the string contains the word "World", ignoring case?

if($mystring =~ m/World/i) { print "Yes"; }

Yes, it does. The pattern modifier i immediately after the trailing / changes the match to be case-insensitive.

I want "Hello world!" to be changed to "Hello mom!" instead.

$mystring =~ s/world/mom/;
print $mystring;

Prints "Hello mom!". The substitution operator s/// replaces the pattern between the s/ and the middle /, with the pattern between the middle / and last /. In this case, "world" is replaced with the word "mom".

Now change "Hello mom!" to say "Goodby mom!".

$mystring =~ s/hello/Goodbye/;
print $mystring;

This does not substitute, and prints "Hello mom!" as before. By default, the search is case sensitive. As before, use the pattern modifier i immediately after the trailing / to make the search case-insensitive.

Okay, ignoring case, change "Hello mom!" to say "Goodby mom!".

$mystring =~ s/hello/Goodbye/i;
print $mystring;

Prints "Goodby mom!".

Section 2: Extracting substrings

I want to see if my string contains a digit.

$mystring = "[2004/04/13] The date of this article.";
if($mystring =~ m/\d/) { print "Yes"; }

Prints "Yes". The pattern \d matches any single digit. In this case, the search will finish as soon as it reads the "2". Searching always goes left to right.

Huh? Why doesn't "\d" match the exact characters '\' and 'd'?

This is because Perl uses characters from the alphabet to also match things with special meaning, like digits. To differentiate between matching a regular character and something else, the character is immediately preceded by a backslash. Therefore, whenever you read '\' followed by any character, you treat the two together as one symbol. For example, '\d' means digit, '\w' means alphanumeric characters including '_', '\/' means forward slash, and '\\' means match a single backslash. Preceding a character with a '\' is called escaping, and the '\' together with its character is called an escape sequence.

Okay, how do I return the first matching digit from my string?

$mystring = "[2004/04/13] The date of this article.";
if($mystring =~ m/(\d)/) {
	print "The first digit is $1.";
}

Prints "The first digit is 2." In order to designate a pattern for extraction, one places parenthesis around the pattern. If the pattern is matched, it is returned in the Perl special variable called $1. If there are multiple parenthesized expressions, then they will be in variables $1, $2, $3, etc.

Huh? Why doesn't '(' and ')' match the parenthesis symbols exactly?

This is because the designers of regular expressions felt that some constructs are so common that they should use unescaped characters to represent them. Besides parentheses, there are a number of other characters that have special meanings when unescaped, and these are called metacharacters. To match parenthesis characters or other metacharacters, you have to escape them like '$' and '$'. They designed it for their convenience, not to make it easy to learn.

Okay, how do I extract a complete number, like the year?

$mystring = "[2004/04/13] The date of this article.";
if($mystring =~ m/(\d+)/) {
	print "The first number is $1.";
}

Prints "The first number is 2004." First, when one says "complete number", one really means a grouping of one or more digits. The pattern quantifier + matches one or more of the pattern that immediately precedes it, in this case, the \d. The search will finish as soon as it reads the "2004".

How do I print all the numbers from the string?

$mystring = "[2004/04/13] The date of this article.";
while($mystring =~ m/(\d+)/g) {
	print "Found number $1.";
}

Prints "Found number 2004. Found number 04. Found number 13. ". This introduces another pattern modifier g, which tells Perl to do a global search on the string. In other words, search the whole string from left to write.

How do I get all the numbers from the string into an array instead?

$mystring = "[2004/04/13] The date of this article.";
@myarray = ($mystring =~ m/(\d+)/g);
print join(",", @myarray);

Prints "2004,04,13". This does the same thing as before, except assigns the returned values from the pattern search into myarray.

Section 3: Common tasks

How do I extract everything between a the words "start" and "end"?

$mystring = "The start text always precedes the end of the end text.";
if($mystring =~ m/start(.*)end/) {
	print $1;
}

Prints text always precedes the end of the . The pattern .* is two different metacharacters that tell Perl to match everything between the start and end. Specifically, the metacharacter . means match any symbol except new line. The pattern quantifier * means match zero or more of the preceding symbol.

That isn't exactly what I expected. How do I extract everything between "start" and the first "end" encountered?

$mystring = "The start text always precedes the end of the end text.";
if($mystring =~ m/start(.*?)end/) {
	print $1;
}

Prints text always precedes the . By default, the quantifiers are greedy. This means that when you say.*, Perl matches every character (except new line) all the way to the end of the string, and then works backward until it finds end. To make the pattern quantifier miserly, you use the pattern quantifier limiter ?. This tells Perl to match as few as possible of the preceding symbol before continuing to the next part of the pattern.

Conclusion

Regular expressions in Perl are very powerful, and there are many ways to do the same thing. I hope you find this page useful to get started in regular expressions. Hopefully, now you can read the specifications and get more out of it.

Perl Book Recommendations

Perl Black Book: The Most Comprehensive Perl Reference Available Today - This book takes a really hands-on, task-oriented approach, and has lots of examples. I would highly recommend it.
Perl in a Nutshell - This book concisely summarizes Perl features

As an experienced, non-Perl programmer, I have been able to get by with the above two books, thecomp.lang.perl newsgroup, and the perldoc documentation. The first book I use when I need some example code to get something working quickly, and the second book I use for reference when I need to look up some regular expression syntax or a specific function call. The Nutshell book is easier to use on my desk as a reference, because it is lightweight. However, if I were to own one book, I would own the Perl Black Book. Neither of these books is for novice programmers who don't understand things like control structures and functions.