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Chapter 6. Pattern Matching

Contents:

Introduction

Copying and Substituting Simultaneously

Matching Letters

Matching Words

Commenting Regular Expressions

Finding the Nth Occurrence of a Match

Matching Within Multiple Lines

Reading Records with a Separator

Extracting a Range of Lines

Matching Shell Globs as Regular Expressions

Speeding Up Interpolated Matches

Testing for a Valid Pattern

Honoring Locale Settings in Regular Expressions

Approximate Matching

Matching from Where the Last Pattern Left Off

Greedy and Non-Greedy Matches

Detecting Doubled Words

Matching Nested Patterns

Expressing AND, OR, and NOT in a Single Pattern

Matching a Valid Mail Address

Matching Abbreviations

Program: urlify

Program: tcgrep

Regular Expression Grab Bag

Sir Herbert Read, The Meaning of Art

[Art is] pattern informed by sensibility.

6.0. Introduction

Most modern programming languages
offer primitive pattern-matching tools, usually through an extra
library. In contrast, Perl's patterns are integrated directly into
the language core. Perl's pattern matching boasts features not found
elsewhere, features that encourage a whole different way of looking
at data. Just as chess players see patterns in the board positions
that their pieces control, Perl adepts look at data in terms of
patterns. These patterns, expressed in the intensely symbolic
notation of regular expressions,[8] provide access to powerful algorithms
normally available only to scholars of computer science.

[8]Technically, Perl's
patterns far exceed the capabilities of mere regular
expressions as that term is formally used in computing
theory.

"If this pattern matching thing is so powerful and so fantastic," you
may be asking, "why don't you have a hundred different recipes on
regular expressions in this chapter?" Regular expressions are the
natural solution to many problems involving numbers, strings, dates,
web documents, mail addresses, and almost everything else in this
book; we use pattern matching over 100 times in other chapters. This
chapter mostly presents recipes in which pattern matching forms part
of the questions, not just part of the answers.

Perl's extensive and integrated support for regular expressions means
that you not only have features available that you won't find in any
other language, but you have new ways of using them, too. Programmers
new to Perl often look for functions like these:

match( $string, $pattern );
subst( $string, $pattern, $replacement );

but matching and substituting are such common tasks that they merit
their own notation:

$meadow =~ m/sheep/; # True if $meadow contains "sheep"
$meadow !~ m/sheep/; # True if $meadow doesn't contain "sheep"
$meadow =~ s/old/new/; # Replace "old" with "new" in $meadow

Pattern matching isn't like direct string comparison, even at its
simplest level; it's more like string searching with mutant wildcards
on steroids. Without anchors, the position where the match occurs can
float freely throughout the string. Any of the following lines would
also be matched by the expression $meadow
=~ /ovine/, giving false
positives when looking for lost sheep:

Fine bovines demand fine toreadors.
Muskoxen are a polar ovibovine species.
Grooviness went out of fashion decades ago.

Sometimes they're right in front of you but they still don't match:

Ovines are found typically in oviaries.

The problem is that while you are probably thinking in some human
language, the pattern-matching engine most assuredly is not. When the
engine is presented with the pattern /ovine/ and a
string to match against, it searches the string for an
"o" that is immediately followed by a
"v", then by an "i", then by an
"n", and then finally by an
"e". What comes before or after that sequence
doesn't matter. Additionally, those letters are matched
case-sensitively. That's why it didn't find
"Ovines", since that string starts with a capital
letter.

As you find your patterns matching some strings you don't want them
to match and not matching other strings that you do want them to
match, you start embellishing. When looking for nothing but sheep,
you probably want to match a pattern more like this:

if ($meadow =~ /\bovines?\b/i) { print "Here be sheep!" }

Don't be tricked by the phantom cow lurking in that
string—that's not a bovine. It's an ovine with a
\b in front, which matches at a word boundary
only.[9] The s? indicates an
optional "s" so we can find one or more ovines.
The trailing /i makes the whole pattern match
case-insensitive.

[9]For Perl's idea of what defines a
"word."

As you see, certain character sequences have special meaning to the
pattern-matching engine, often standing in for several possible
literal characters. These so-called metacharacters let you do such
things as restrict the pattern to the start or end of the string,
give alternatives for parts of a pattern, allow repetition and
wildcarding, and remember part of the matching substring for use
later in the pattern or in code.

Learning the syntax of pattern matching isn't as daunting as it might
appear. Sure, there are a lot of symbols, but each has a reason for
existing. Regular expressions aren't random jumbles of
punctuation—they're carefully thought-out jumbles of
punctuation! If you forget one, you can always look it up. Summary
tables are included in Programming Perl,
Learning Perl, Mastering Regular
Expressions, and the perlre(1) and
perlop(1) manpages included with every Perl
installation.

6.0.2. The Tricky Bits

Greed is the principle that if
a standard quantifier (such as *) can match a
varying number of times, it matches as long a substring as it can.
This is explained in Recipe 6.15.

Eagerness is the notion that the leftmost match wins. The engine is
eager to return you a match as quickly as possible, sometimes even
before you are expecting it. Consider the match
"Fred" =~
/x*/. If asked to explain this in plain language,
you might say "Does the string "Fred" contain any
x's?" If so, you might be surprised to learn that
it seems to. That's because /x*/ doesn't truly
mean "any x's," unless your idea of "any" includes
nothing at all. Formally, it means zero or more
of them, and here zero sufficed for the eager matcher.

A more illustrative example of eagerness would be the following:

$string = "good food";
$string =~ s/o*/e/;

Can you guess which of the following is in $string
after that substitution?

good food
geod food
geed food
geed feed
ged food
ged fed
egood food

The correct answer is the last one, because the earliest point at
which zero or more occurrences of "o" could be
found was right at the beginning of the string. Surprised? Regular
expressions can do that to you if you're unfamiliar with their
semantics.

Here's another example of where greed takes a back seat to eagerness:

$ echo longest | perl -ne 'print "$&\n" if /long|longer|longest/'
long

That's because Perl uses
what's called a traditional NFA,[10] a
non-deterministic finite automaton. This kind of matching engine is
not guaranteed to return the longest overall
match, just the first match. You might think of Perl's greed as being
left-to-right directed, not globally greedy.

[10]As opposed to a
POSIX-style NFA. See Mastering Regular
Expressions for the differences.

NFAs can be slow, but significant performance gains can be made by
rewriting the patterns to exploit how the particular NFA
implementation runs. This is a major part of Jeffrey Friedl's book,
Mastering Regular Expressions.

The
last and most powerful of the three tricky bits in pattern matching
is backtracking. For a pattern to match, the entire regular
expression must match, not just part of it. So if the beginning of a
pattern containing a quantifier succeeds in a way that causes later
parts in the pattern to fail, the matching engine backs up and tries
to find another match for the beginning part—that's why it's
called backtracking. It means that the engine is going to try
different possibilities, systematically investigating alternate
matches until it finds one that works. In some pattern-matching
implementations, the engine keeps backtracking in case other
submatches make the overall match longer. Perl's matcher doesn't do
that; as soon as one possibility works, it uses that—until and
unless something later in the pattern fails, forcing a backtrack to
retry another possible way of matching. This is discussed in Recipe 6.16.

6.0.3. Pattern-Matching Modifiers

Pattern-matching modifiers are a lot
easier to list and learn than the different metacharacters. Table 6-1 contains a brief summary of them.

Table 6-1. Pattern-matching modifiers and their meanings

/i and
/g are the most commonly used modifiers. The
pattern /ram/i matches "ram",
"RAM", "Ram", and so forth.
Backreferences are checked case-insensitively if this modifier is on;
see Recipe 6.16 for an example. This
case-insensitivity can be made aware of the user's current locale
settings if the use locale
pragma has been invoked.

The /g modifier is
used with s/// to replace every non-overlapping
match, not just the first one. /g is also used
with m// in loops to find (but not replace) every
matching occurrence:

while (m/(\d+)/g) {
print "Found number $1\n";
}

Used on m// in list context, /g
pulls out all matches:

@numbers = m/(\d+)/g;

That finds only non-overlapping matches. You have to be much sneakier
to get overlapping ones by making a zero-width look-ahead with the
(?=...) construct. Because it's zero-width, the
match engine hasn't advanced at all. Within the look-ahead, capturing
parentheses are used to grab the thing anyway. Although we've saved
something, Perl notices we haven't made any forward progress on the
/g, so it bumps us forward one character position.

This shows the difference:

$digits = "123456789";
@nonlap = $digits =~ /(\d\d\d)/g;
@yeslap = $digits =~ /(?=(\d\d\d))/g;
print "Non-overlapping: @nonlap\n";
print "Overlapping: @yeslap\n";
Non-overlapping: 123 456 789
Overlapping: 123 234 345 456 567 678 789

The /s and
/m modifiers are useful when matching strings with
embedded newlines. /s makes dot match
"\n", something it doesn't normally do; it also
makes the match ignore the value of the old, deprecated
$* variable. /m makes
^ and $ match after and before
"\n", respectively. They are useful with paragraph
slurping mode as explained in the Introduction to Chapter 8, and in Recipe 6.6.

The /e modifier is
used on replacements so that the righthand part is run as code and
its return value is used as the replacement string.
s/(\d+)/sprintf("%#x", $1)/ge
converts all numbers into hex, changing, for example,
2581 into
0xb23.

Because
different countries have different ideas of what constitutes an
alphabet, the POSIX standard provides systems (and thus programs)
with a standard way of representing alphabets, character set
ordering, and so on. Perl gives you access to some of these through
the use locale pragma; see the
perllocale manpage for more information. When
use locale is in effect, the
\w character class includes accented and other
exotic characters. The case-changing \u,
\U, \l, and
\L (and the corresponding uc,
ucfirst, etc. functions) escapes also respect
use locale, so σ
will be turned into Σ with \u if the
locale says it should. (This only matters in 8-bit encodings, such as
ISO 8859-7 for the Greek character set. If those characters had been
in Unicode, case translation would always apply, irrespective of
current locale setting.)

6.0.4. Special Variables

Perl sets special variables as the result of
certain matches: $1, $2,
$3, and so on ad infinitum
are set when a pattern contains capturing parentheses within parts of
the pattern. Each open parenthesis as you read left to right in the
pattern begins filling a new, numbered variable. The variable
$+ contains the contents of the last backreference
of the last successful match. This helps distinguish which of several
alternate matches was found (for example, if
/(x.*y)|(y.*z)/ matches, $+
contains whichever of $1 or $2
were filled). $& contains the complete text
matched in the last successful pattern match. $`
and $´ are the strings before and after
the successful match, respectively:

$string = "And little lambs eat ivy";
$string =~ /l[^s]*s/;
print "($`) ($&) ($´)\n";
(And ) (little lambs) ( eat ivy)

$`, $&, and
$´ are tempting, but dangerous. Their
very presence anywhere in a program slows down every pattern match
because the engine must populate these variables for every match.
This is true even if you use one of these variables only once, or,
for that matter, if you never use them at all, only mention them.
Using $& is no longer so expensive as the
other two.

A cheaper approach is to use the
substr function in conjunction with the built-in
array variables @- and @+,
first introduced in Perl v5.6. These represent the starting and
ending positions of the last submatches, respectively. The
Nth elements of these two arrays hold the
beginning and ending offset of the Nth submatch.
So $-[1] is the offset where $1
begins, and $+[1] is the offset where it ends;
$-[2] is the offset where $2
begins, and $+[2] is the offset where it ends; and
so on. $-[0] is the offset of the beginning of the
entire match, and $+[0] the offset of the end of
the entire match. (When we say "offset of the end," we mean the
offset to the first character following the end
of whatever matched, so that we can subtract beginning offsets from
end offsets and arrive at the length.)

After a match against some variable $string, the
following equivalencies hold true:

Variable Equivalent
$` substr($string, 0, $-[0])
$& substr($string, $-[0], $+[0] - $-[0])
$´ substr($string, $+[0])
$1 substr($string, $-[1], $+[1] - $-[1])
$2 substr($string, $-[2], $+[2] - $-[2])
$3 substr($string, $-[3], $+[3] - $-[3])

And so on and so forth.

To learn far more about regular expressions than you ever thought
existed, check out Mastering Regular
Expressions, written by Jeffrey Friedl (O'Reilly). This
book is dedicated to explaining regular expressions from a practical
perspective. Not only does it cover general regular expressions and
Perl specials, it also compares and contrasts these with patterns in
other programming languages.

5.17. Program: dutree		6.1. Copying and Substituting Simultaneously

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