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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :


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5.6. Regular Expressions Best Practices and Tips

One needs to be very careful with regular expressions and avoid overcomplexity like a plague. In complex regular expressions surprises are a dozen for dollar for the uninitiated. Even careful testing does not guarantee that you fully understand its behavior. Complex regular expressions  provide for enormous number of ways to shoot yourself in the foot!

This material is adapted from O'Reilly Network Five Habits for Successful Regular Expressions

Use extended syntax for complex patterns

Consider the following regular expression to match a U.S. phone number:

 \(?\d{3}\)?\s?\d{3}[-.]\d{4} 

This regex matches phone numbers like "(973)555-4000". Ask yourself if the regex would match "973-555-4000" or "555-4000". The answer is no in both cases. Writing this pattern on one line conceals both flaws and design decisions. The area code actually can't be omitted and form 973-555-4000 will not be accepted.

Spreading the pattern out over several lines makes the flaws more visible and the necessary modifications easier. In Perl using extended syntax (option x) we can re-write this expression to accept the second form as following:

 /  
    \(?     # optional parentheses
      \d{3} # area code required
    \)?     # optional parentheses
    [-\s.]? # separator is either a dash, a space, or a period.
      \d{3} # 3-digit prefix
    [-.]    # another separator
      \d{4} # 4-digit line number
/x 

The rewritten regex now has an optional separator after the area code so that it matches phone 973-555-4000 as well as (973)555-4000. The area code is still required.

However, a new programmer who wants to make the area code optional can quickly see that it is not optional now and might change the code to use separate regex for each of three major cases instead of merging them like we did. So much for our optimization ;-). Readable code helps immensely but in no way it is a substitute for good design.

Write Tests for each and every complex regex

There are three levels of testing, each adding a higher level of reliability to your code. First, you need to think hard about what you want to match and whether you can deal with false matches. Second, you need to test the regex on example data. Third, you need to formalize the tests into a test suite.

Deciding what to match is a trade-off between making false matches and missing valid matches. If your regex is too strict, it will miss valid matches. If it is too loose, it will generate false matches. Once the regex is released into live code, you probably will not notice either way. Consider the phone regex example above; it would match the text "800-555=0355". False matches are hard to catch, so it's important to plan ahead and test.

Sticking with the phone number example, if you are validating a phone number on a web form, you may settle for ten digits in any format. However, if you are trying to extract phone numbers from a large amount of text, you might want to be more strict to avoid a unacceptable numbers due to  false matches.

When thinking about what you want to match, write down example cases. Then write some code that tests your regular expression against the example cases. Any complicated regular expression is best written in a small test program, as the examples below demonstrate:

In Perl:

#!/usr/bin/perl

my @tests = ( "314-555-4000",
              "800-555-4400",
	      "(314)555-4000",
              "314.555.4000",
              "555-4000",
              "aasdklfjklas",
              "1234-123-12345"          
            );
foreach my $test (@tests) {
    if ( $test =~ m/
       \(?     # optional parentheses
       \d{3} # area code required
       \)?     # optional parentheses
       [-\s.]? # separator is either a dash, a space, or a period.
       \d{3} # 3-digit prefix
       [-\s.]  # another separator
       \d{4} # 4-digit line number/x ) {
        print "Matched on $test\n";
    } else {
        print "Failed match on $test\n";
    } # if
} # foreach

Running the test script exposes yet another problem in the phone number regex: it matched "1234-123-12345". That demonstrates the key principle of text selection: include both tests that you expect to fail (they may succeed) as well as those you expect to match (they may not)

Ideally, you preseve this test as part of the test suite for your entire program. Even if you do not have a test suite already, your regular expression tests are a good foundation for a suite, and now is the perfect opportunity to start on one. Even if now is not the right time (really, it is!), you should make a habit to run your regex tests after every modification. A little extra time here could save you many headaches.

Group the Alternation Operator

The alternation operator (|) has a low precedence. This means that it often alternates over more than the programmer intended. For example, a regex to extract email addresses out of a mail file might look like:

^CC:|To:(.*)

The above attempt is incorrect, but the bugs often go unnoticed. The intent of the above regex is to find lines starting with "CC:" or "To:" and then capture any email addresses on the rest of the line.

Unfortunately, the regex doesn't actually capture anything from lines starting with "CC:" and may capture random text if "To:" appears in the middle of a line. In plain English, the regular expression matches lines beginning with "CC:" and captures nothing, or matches any line containing the text "To:" and then captures the rest of the line. Usually, it will capture plenty of addresses and nobody will notice the failings.

If that were the real intent, you should add parentheses to say it explicitly, like this:

(^CC:)|(To:(.*))

However, the real intent of the regex i to match lines starting with "CC:" or "To:" and then capture the rest of the line. The following regex does that:

^(CC:|To:)(.*)

This is a common and hard-to-catch bug. If you develop the habit of wrapping your alternations in parentheses (or non-capturing parentheses -- (?:)) you can avoid this error.

Use Lazy Quantifiers

Most people avoid using the lazy quantifiers *?, +?, and ??, even though they are easy to understand and make many regular expressions easier to write.

Lazy quantifiers match as little text as possible while still aiding the success of the overall match. If you write foo(.*?)bar, the quantifier will stop matching the first time it sees "bar", not the last time. This may be important if you are trying to capture ###; in the text foo###bar+++bar. A regular quantifier would have captured ###bar+++.

Let's say you want to capture all of the phone numbers from an HTML file. You could use the phone number regular expression example we discussed earlier in this article. However, if you know that the file contains all of the phone numbers in the first column of a table, you can write a much simpler regex using lazy quantifiers:

<tr><td>(.+?)<td>

Many beginning regular expression programmers avoid lazy quantifiers with negated character classes. They write the above code as:

<tr><td>([^>]+)</td>

That works in this case, but leads to trouble if the text you are trying to capture contains common characters from your delimiter (in this case, </td>). If you use lazy quantifiers, you will spend less time kludging character classes and produce clearer regular expressions.

Lazy quantifiers are most valuable when you know the structure surrounding the text you want to capture.

Use Alternative Delimiters

Perl allow you to use any non-alphanumeric or whitespace character as a delimiter. If you switch to a new delimiter, you can avoid having to escape the forward slashes when you are trying to match URLs or HTML tags such as "http://" or "<br />".

For example:

/http:\/\/(\S)*/

could be rewritten as:

#http://(\S)*#

Common delimiters are #, !, |. If you use square brackets, angle brackets, or curly braces, the opening and closing brackets must match. Here are some common uses of delimiters:

#…# !…! {…}
s|…|…| s[…][…] s<…>/…/

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