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There are two reasons regular expressions are so hard to read and are so error prone. One, the syntax is terse. Two, programmers ignore all normal programming practices. This talk reintroduces white space, structure, and basic verification/testing and then calls them "Best Practices."
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Regular Expression Best PracticesTony Stubblebine
www.stubbleblog.com
@tonystubblebine
Tabbed indentation is a sin but this isn't?
$string =~ s<(?:http://(?:(?:(?:(?:(?:[a-zA-Z\d](?:(?:[a-zA-Z\d]|-)*[a-zA-Z\d])?)\.
)*(?:[a-zA-Z](?:(?:[a-zA-Z\d]|-)*[a-zA-Z\d])?))|(?:(?:\d+)(?:\.(?:\d+)
){3}))(?::(?:\d+))?)(?:/(?:(?:(?:(?:[a-zA-Z\d$\-_.+!*'(),]|(?:%[a-fA-F
\d]{2}))|[;:@&=])*)(?:/(?:(?:(?:[a-zA-Z\d$\-_.+!*'(),]|(?:%[a-fA-F\d]{
2}))|[;:@&=])*))*)(?:\?(?:(?:(?:[a-zA-Z\d$\-_.+!*'(),]|(?:%[a-fA-F\d]{
2}))|[;:@&=])*))?)?)|(?:ftp://(?:(?:(?:(?:(?:[a-zA-Z\d$\-_.+!*'(),]|(?
:%[a-fA-F\d]{2}))|[;?&=])*)(?::(?:(?:(?:[a-zA-Z\d$\-_.+!*'(),]|(?:%[a-
fA-F\d]{2}))|[;?&=])*))?@)?(?:(?:(?:(?:(?:[a-zA-Z\d](?:(?:[a-zA-Z\d]|-
)*[a-zA-Z\d])?)\.)*(?:[a-zA-Z](?:(?:[a-zA-Z\d]|-)*[a-zA-Z\d])?))|(?:(?
:\d+)(?:\.(?:\d+)){3}))(?::(?:\d+))?))(?:/(?:(?:(?:(?:[a-zA-Z\d$\-_.+!
*'(),]|(?:%[a-fA-F\d]{2}))|[?:@&=])*)(?:/(?:(?:(?:[a-zA-Z\d$\-_.+!*'()
,]|(?:%[a-fA-F\d]{2}))|[?:@&=])*))*)(?:;type=[AIDaid])?)?)|(?:news:(?:
...................
Abigail, comp.lang.perl.misc, http://aspn.activestate.com/ASPN/Cookbook/Rx/Recipe/59864
Best Practices for Any Programming
There are programming fundamentals that are routinely ignored by regular expression writers.
Put a line break after statements and space between expressions.
Throw in a comment or two. Use subroutines and modules to show structure
and avoid duplication. Test.
Good Code# Given a URL/URI, fetches it.
# Returns an HTTP::Response object.
sub get {
my $self = shift; my $uri = shift;
$uri = $self->base
? URI->new_abs( $uri, $self->base )
: URI->new( $uri );
return $self->SUPER::get( $uri->as_string, @_ );
}
What if we didn't include documentation or whitespace?
sub get{my$self=shift;my$uri=shift;$uri=$self->base?URI->new_abs($uri,$self->base):URI->new($uri);return$self->SUPER::get($uri->as_string,@_);}
What if we were also as terse as possible?
So: No documentation No whitespace One character variable and method names
We'd have a regular expression.
sub g{my($s,$u)=@_;$u=$s->b?U-> n($u,$s->b):U->q($u);return$s->SUPER::g($u->a,@_);}
What do we want from best practices?
Practices that maximize desired goals in certain applications.
Goals of regex best practices: Maintainability Correctness Development Speed
#1: Use Extended Whitespace
Add indentation, newlines, and comments to regular expressions
Usage /x: m/regex/x
# Look for green or red foxes
$text =~ /(green | red)
\s
fox (es)?
# Allow more than one
/x;
Extended Whitespace Gotchas
• Must explicitly ask to match a space with \s or \<SPACE>
• Must escape pound signs, \#
Before
What does this match?
$text =~ m/^([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])\.([01]?\d\d?|2[0-4]\d|25[0-5])$/;
After
$text =~ m/
# Match IP addresses like 169.146.10.45
^ # Start of string
([01]?\d\d?|2[0-4]\d|25[0-5])
# Number, 0-255
\.([01]?\d\d?|2[0-4]\d|25[0-5])
# 0-255
\.([01]?\d\d?|2[0-4]\d|25[0-5])
# 0-255
\.([01]?\d\d?|2[0-4]\d|25[0-5])
# 0-255
$/x;
#2 Test
You don't know your data. And you have a typo in your regex. Guaranteed surprises on both fronts.
Fun Gotcha
What file does this code open?
$file =
"/etc/passwd\0/var/www/index.html";
if ( $file =~ m/^ .* \.html/x ) {
open (FILE, "$file);
}
Typical Gotcha
This matches foo.gif
But also... foojpg and jpg.doc
# match image files
m/ \. gif | jpg | jpeg | png $/x
Test framework
Write your regular expressions in a place where you can test them.
Build up a list of positive and negative matches Include list in your documentation, ex:
# matches 800-555-1212 but not
# 800.555.1212 or 800-BETS-OFF
Hackers Test Framework
Your “framework” could be this simple:
foreach my $test (@tests) {
# looks like an image file?
if (
$test =~ m/ \. gif | jpg | jpeg | png $/x ) {
print "Matched on $test\n";
} else {
print "Failed match on $test\n";
}
}
Real Tests Are Bettermy @match = ("foo.gif", "foo.bar.jpg", "bar_foo.gif.jpg.png");
my @fail = ("gif.foo", "foo.gif.", "foopng", "foo.jpeg.bar");
sub match {
return $_[0] =~ m/ \. gif | jpg | jpeg | png $/x;
}
foreach my $test (@match) {
ok( match($test), "$test matches");
}
foreach my $test (@fail) {
ok( !match($test), "$test fails to match");
}
#3 Use Structure
... as a slow-witted human being I have a very small head and I had better learn to live with it and to respect my limitations and give them full credit, rather than to try to ignore them, for the latter vain effort will be punished by failure.
~ Edsger Dijkstra
Breaking up an email regex
We can write an email regex that looks like this:
m/$user\@$domain/
Build your regexes from smaller regexes like this:
$user = "\w+";
$domain = qr/\w+\.(\w+\.)*\w\w\w?/i;
Use Post Processing
It's easier to say a number is <= 255 in code than it is as a regular expression.
# IP Address check
$ip =~ m/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
foreach my $num ($1, $2, $3, $4) {
$failure++ unless $num < 256;
}
#4. Good habits
Regex are hard to debug, so avoid errors. Error avoidance habits:
Group alternations with parentheses Use lazy quantifiers Don't use regular expressions
Group Alternations
Group your alternations. In this regex, the dot and end of string ($) are not part of your alternation.
m/ \. (gif | jpg | jpeg | png) $/x
Use Lazy Quantifiers
Use lazy quantifiers. It's easier to say when to stop.
<td>.*?</td>
Lazy Quantifiers...
Compare that to
#Matches too much
$text = "<td>foo</td><td>bar</td>";
$text =~ m!<td>.*</td>!;
#Matches too little
$text = "<td>foo <b>bar</b> </td>";
$text = m/<td>[^<]*/;
Don't use regular expressions
Regular expressions don't deal well with nesting
$text = "<td> foo <table><tr><td>bar</td>...";
$text =~ m!<td> .*? </td>!;
Use something better an HTML or XML parsing library instead.
Don't use regular expressions
Regular expressions don't deal well with nesting
$text = "<td> foo <table><tr><td>bar</td>...";
$text =~ m!<td> .*? </td>!;
Use something better an HTML or XML parsing library instead.
#5. Optimize Last
It's more common for regular expressions to be broken then to be slow
Optimize last. Start with the quantifiers
Optimizing Quantifiers
# This is slow because the match backtracks from the end
# of the file
$text = "M1 text i'm looking for M2 thousand more characters to come...";
$text =~ m/M1 (.*) M2/s;
# This is slow because the match looks for </body> at
# (nearly) every position.
$html =~ m!<body> (.*?) </body>!xs;
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