perlretut - Perl regular expressions tutorial
This page provides a basic tutorial on understanding, creating
and using regular expressions in Perl. It serves as
a complement to the reference page on regular expressions
perlre. Regular expressions are an integral part of the
"m//", "s///", "qr//" and "split" operators and so this
tutorial also overlaps with "Regexp Quote-Like Operators"
in perlop and "split" in perlfunc.
Perl is widely renowned for excellence in text processing,
and regular expressions are one of the big factors behind
this fame. Perl regular expressions display an efficiency
and flexibility unknown in most other computer languages.
Mastering even the basics of regular expressions will
allow you to manipulate text with surprising ease.
What is a regular expression? A regular expression is
simply a string that describes a pattern. Patterns are in
common use these days; examples are the patterns typed
into a search engine to find web pages and the patterns
used to list files in a directory, e.g., "ls *.txt" or
"dir *.*". In Perl, the patterns described by regular
expressions are used to search strings, extract desired
parts of strings, and to do search and replace operations.
Regular expressions have the undeserved reputation of
being abstract and difficult to understand. Regular
expressions are constructed using simple concepts like
conditionals and loops and are no more difficult to understand
than the corresponding "if" conditionals and "while"
loops in the Perl language itself. In fact, the main
challenge in learning regular expressions is just getting
used to the terse notation used to express these concepts.
This tutorial flattens the learning curve by discussing
regular expression concepts, along with their notation,
one at a time and with many examples. The first part of
the tutorial will progress from the simplest word searches
to the basic regular expression concepts. If you master
the first part, you will have all the tools needed to
solve about 98% of your needs. The second part of the
tutorial is for those comfortable with the basics and hungry
for more power tools. It discusses the more advanced
regular expression operators and introduces the latest
cutting edge innovations in 5.6.0.
A note: to save time, 'regular expression' is often abbreviated
as regexp or regex. Regexp is a more natural
abbreviation than regex, but is harder to pronounce. The
Perl pod documentation is evenly split on regexp vs regex;
in Perl, there is more than one way to abbreviate it.
We'll use regexp in this tutorial.
Part 1: The basics
Simple word matching
The simplest regexp is simply a word, or more generally, a
string of characters. A regexp consisting of a word
matches any string that contains that word:
"Hello World" =~ /World/; # matches
What is this perl statement all about? "Hello World" is a
simple double quoted string. "World" is the regular
expression and the "//" enclosing "/World/" tells perl to
search a string for a match. The operator "=~" associates
the string with the regexp match and produces a true value
if the regexp matched, or false if the regexp did not
match. In our case, "World" matches the second word in
"Hello World", so the expression is true. Expressions
like this are useful in conditionals:
if ("Hello World" =~ /World/) {
print "It matches0;
}
else {
print "It doesn't match0;
}
There are useful variations on this theme. The sense of
the match can be reversed by using "!~" operator:
if ("Hello World" !~ /World/) {
print "It doesn't match0;
}
else {
print "It matches0;
}
The literal string in the regexp can be replaced by a
variable:
$greeting = "World";
if ("Hello World" =~ /$greeting/) {
print "It matches0;
}
else {
print "It doesn't match0;
}
If you're matching against the special default variable
$_, the "$_ =~" part can be omitted:
$_ = "Hello World";
if (/World/) {
print "It matches0;
}
else {
print "It doesn't match0;
}
And finally, the "//" default delimiters for a match can
be changed to arbitrary delimiters by putting an 'm' out
front:
"Hello World" =~ m!World!; # matches, delimited by
'!'
"Hello World" =~ m{World}; # matches, note the
matching '{}'
"/usr/bin/perl" =~ m"/perl"; # matches after
'/usr/bin',
# '/' becomes an ordinary
char
"/World/", "m!World!", and "m{World}" all represent the
same thing. When, e.g., "" is used as a delimiter, the
forward slash '/' becomes an ordinary character and can be
used in a regexp without trouble.
Let's consider how different regexps would match "Hello
World":
"Hello World" =~ /world/; # doesn't match
"Hello World" =~ /o W/; # matches
"Hello World" =~ /oW/; # doesn't match
"Hello World" =~ /World /; # doesn't match
The first regexp "world" doesn't match because regexps are
case-sensitive. The second regexp matches because the
substring 'o W' occurs in the string "Hello World" . The
space character ' ' is treated like any other character in
a regexp and is needed to match in this case. The lack of
a space character is the reason the third regexp 'oW'
doesn't match. The fourth regexp 'World ' doesn't match
because there is a space at the end of the regexp, but not
at the end of the string. The lesson here is that regexps
must match a part of the string exactly in order for the
statement to be true.
If a regexp matches in more than one place in the string,
perl will always match at the earliest possible point in
the string:
"Hello World" =~ /o/; # matches 'o' in 'Hello'
"That hat is red" =~ /hat/; # matches 'hat' in 'That'
With respect to character matching, there are a few more
points you need to know about. First of all, not all
characters can be used 'as is' in a match. Some characters,
called metacharacters, are reserved for use in regexp
notation. The metacharacters are
{}[]()^$.|*+?
The significance of each of these will be explained in the
rest of the tutorial, but for now, it is important only to
know that a metacharacter can be matched by putting a
backslash before it:
"2+2=4" =~ /2+2/; # doesn't match, + is a metacharacter
"2+2=4" =~ /2+2/; # matches, + is treated like an
ordinary +
"The interval is [0,1)." =~ /[0,1)./ # is a syntax
error!
"The interval is [0,1)." =~ /./ # matches
"/usr/bin/perl" =~ /usrbinperl/; # matches
In the last regexp, the forward slash '/' is also backslashed,
because it is used to delimit the regexp. This
can lead to LTS (leaning toothpick syndrome), however, and
it is often more readable to change delimiters.
"/usr/bin/perl" =~ m!/usr/bin/perl!; # easier to
read
The backslash character '' is a metacharacter itself and
needs to be backslashed:
'C:WIN32' =~ /C:\WIN/; # matches
In addition to the metacharacters, there are some ASCII
characters which don't have printable character equivalents
and are instead represented by escape sequences.
C"mfor axcarriagerreturnoand ""bfor0afbell.neIfiyour
"
string is better thought of as a sequence of arbitrary
bytes, the octal escape sequence, e.g., " 33", or hexadecimal
escape sequence, e.g., "B" may be a more natural
representation for your bytes. Here are some examples
of escapes:
"10002000" =~ m(02) # matches
"10000000" =~ /000/ # matches
"10002000" =~ / 002/ # doesn't match, "0" ne " 00"
"cat" =~ /14314/ # matches, but a weird way to
spell cat
If you've been around Perl a while, all this talk of
escape sequences may seem familiar. Similar escape
sequences are used in double-quoted strings and in fact
the regexps in Perl are mostly treated as double-quoted
strings. This means that variables can be used in regexps
as well. Just like double-quoted strings, the values of
the variables in the regexp will be substituted in before
the regexp is evaluated for matching purposes. So we
have:
$foo = 'house';
'housecat' =~ /$foo/; # matches
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matches
So far, so good. With the knowledge above you can already
perform searches with just about any literal string regexp
you can dream up. Here is a very simple emulation of the
Unix grep program:
% cat > simple_grep
#!/usr/bin/perl
$regexp = shift;
while (<>) {
print if /$regexp/;
}
^D
% chmod +x simple_grep
% simple_grep abba /usr/dict/words
Babbage
cabbage
cabbages
sabbath
Sabbathize
Sabbathizes
sabbatical
scabbard
scabbards
This program is easy to understand. "#!/usr/bin/perl" is
the standard way to invoke a perl program from the shell.
"$regexp = shift;" saves the first command line argument
as the regexp to be used, leaving the rest of the command
line arguments to be treated as files. "while (<>)"
loops over all the lines in all the files. For each line,
"print if /$regexp/;" prints the line if the regexp
matches the line. In this line, both "print" and "/$regexp/"
use the default variable $_ implicitly.
With all of the regexps above, if the regexp matched anywhere
in the string, it was considered a match. Sometimes,
however, we'd like to specify where in the string
the regexp should try to match. To do this, we would use
the anchor metacharacters "^" and "$". The anchor "^"
means match at the beginning of the string and the anchor
"$" means match at the end of the string, or before a newline
at the end of the string. Here is how they are used:
"housekeeper" =~ /keeper/; # matches
"housekeeper" =~ /^keeper/; # doesn't match
"housekeeper" =~ /keeper$/; # matches
"housekeeper0 =~ /keeper$/; # matches
The second regexp doesn't match because "^" constrains
"keeper" to match only at the beginning of the string, but
"housekeeper" has keeper starting in the middle. The
third regexp does match, since the "$" constrains "keeper"
to match only at the end of the string.
When both "^" and "$" are used at the same time, the regexp
has to match both the beginning and the end of the
string, i.e., the regexp matches the whole string. Consider
"keeper" =~ /^keep$/; # doesn't match
"keeper" =~ /^keeper$/; # matches
"" =~ /^$/; # ^$ matches an empty
string
The first regexp doesn't match because the string has more
to it than "keep". Since the second regexp is exactly the
string, it matches. Using both "^" and "$" in a regexp
forces the complete string to match, so it gives you complete
control over which strings match and which don't.
Suppose you are looking for a fellow named bert, off in a
string by himself:
"dogbert" =~ /bert/; # matches, but not what you
want
"dilbert" =~ /^bert/; # doesn't match, but ..
"bertram" =~ /^bert/; # matches, so still not good
enough
"bertram" =~ /^bert$/; # doesn't match, good
"dilbert" =~ /^bert$/; # doesn't match, good
"bert" =~ /^bert$/; # matches, perfect
Of course, in the case of a literal string, one could just
as easily use the string equivalence "$string eq 'bert'"
and it would be more efficient. The "^...$" regexp
really becomes useful when we add in the more powerful
regexp tools below.
Using character classes [Toc] [Back]
Although one can already do quite a lot with the literal
string regexps above, we've only scratched the surface of
regular expression technology. In this and subsequent
sections we will introduce regexp concepts (and associated
metacharacter notations) that will allow a regexp to not
just represent a single character sequence, but a whole
class of them.
One such concept is that of a character class. A character
class allows a set of possible characters, rather than
just a single character, to match at a particular point in
a regexp. Character classes are denoted by brackets
"[...]", with the set of characters to be possibly matched
inside. Here are some examples:
/cat/; # matches 'cat'
/[bcr]at/; # matches 'bat, 'cat', or 'rat'
/item[0123456789]/; # matches 'item0' or ... or
'item9'
"abc" =~ /[cab]/; # matches 'a'
In the last statement, even though 'c' is the first character
in the class, 'a' matches because the first character
position in the string is the earliest point at which
the regexp can match.
/[yY][eE][sS]/; # match 'yes' in a case-insensitive way
# 'yes', 'Yes', 'YES', etc.
This regexp displays a common task: perform a case-insensitive
match. Perl provides away of avoiding all those
brackets by simply appending an 'i' to the end of the
match. Then "/[yY][eE][sS]/;" can be rewritten as
"/yes/i;". The 'i' stands for case-insensitive and is an
example of a modifier of the matching operation. We will
meet other modifiers later in the tutorial.
We saw in the section above that there were ordinary characters,
which represented themselves, and special characters,
which needed a backslash "
selves. The same is true in a character class, but the
sets of ordinary and special characters inside a character
class are different than those outside a character class.
The special characters for a character class are "-]$".
"]" is special because it denotes the end of a character
class. "$" is special because it denotes a scalar variable.
"
sequences, just like above. Here is how the special characters
"]$
/[]c]def/; # matches ']def' or 'cdef'
$x = 'bcr';
/[$x]at/; # matches 'bat', 'cat', or 'rat'
/[]at/; # matches '$at' or 'xat'
/[\$x]at/; # matches 't', 'bat, 'cat', or 'rat'
The last two are a little tricky. in "[]", the backslash
protects the dollar sign, so the character class has
two members "$" and "x". In "[\$x]", the backslash is
protected, so $x is treated as a variable and substituted
in double quote fashion.
The special character '-' acts as a range operator within
character classes, so that a contiguous set of characters
can be written as a range. With ranges, the unwieldy
"[0123456789]" and "[abc...xyz]" become the svelte "[0-9]"
and "[a-z]". Some examples are
/item[0-9]/; # matches 'item0' or ... or 'item9'
/[0-9bx-z]aa/; # matches '0aa', ..., '9aa',
# 'baa', 'xaa', 'yaa', or 'zaa'
/[0-9a-fA-F]/; # matches a hexadecimal digit
/[0-9a-zA-Z_]/; # matches a "word" character,
# like those in a perl variable name
If '-' is the first or last character in a character
class, it is treated as an ordinary character; "[-ab]",
"[ab-]" and "[a-b]" are all equivalent.
The special character "^" in the first position of a character
class denotes a negated character class, which
matches any character but those in the brackets. Both
"[...]" and "[^...]" must match a character, or the match
fails. Then
/[^a]at/; # doesn't match 'aat' or 'at', but matches
# all other 'bat', 'cat, '0at', '%at', etc.
/[^0-9]/; # matches a non-numeric character
/[a^]at/; # matches 'aat' or '^at'; here '^' is ordinary
Now, even "[0-9]" can be a bother the write multiple
times, so in the interest of saving keystrokes and making
regexps more readable, Perl has several abbreviations for
common character classes:
o is a digit and represents [0-9]
0f] o is a whitespace character and represents [
o 48a word character (alphanumeric or _) and represents
[0-9a-zA-Z_]
o a negated ; it represents any character but a
digit [^0-9]
o is a negated t represents any non-whitespace
character [^
o W is a negated 840
ter [^0
o The period '.' matches any character but "0
The "28
and outside of character classes. Here are some in use:
.
PERLRETUT(1) Perl Programmers Reference Guide PERLRETUT(1)
h
i
s /::/; # matches a hh:mm:ss time format
m /[ # matches any digit or whitespace character
a /96
t # non-word char, followed by a word
char c
h /..rt/; # matches any two chars, followed by
'rt' e
s /end./; # matches 'end.'
a /end[.]/; # same thing, matches 'end.'
b
Bocause a period is a metacharacter, it needs to be
eucaped to mat;h as an ordinary period. Because, for exampne,
"" and "9#0
td think of "[m960
DaMorgan's lawa.
r t
Ay anchor usefcl in basic regexps is the word anchor
"b h
and a non-wardecharacter "14424
t t s
w $x = "H/usccat catenates house and cat";
e $x =~ /;ata; # matches cat in 'housecat'
e $x =~ /#het cat in 'catenates'
n $x =~ /caati
a $x =~ /t n
w t '
Note in the last example, the end of the string is considered
a word boundary.
d u
You might wonder why '.' matches everything but "0 - why
not every character? The reason is that often one is
matching against lines and would like to ignore the newline
characters. For instance, while the string "0 represents
one line, we would like to think of as empty.
Then '
t
e "" =~ /^$/; # matches
r "0 =~ /^$/; # matches, "0 is ignored
"" =~ /./; # doesn't match; it needs a char
"" =~ /^.$/; # doesn't match; it needs a char
"0 =~ /^.$/; # doesn't match; it needs a char other
than "0
"a" =~ /^.$/; # matches
"a0 =~ /^.$/; # matches, ignores the "0
This behavior is convenient, because we usually want to
ignore newlines when we count and match characters in a
line. Sometimes, however, we want to keep track of newlines.
We might even want "^" and "$" to anchor at the
beginning and end of lines within the string, rather than
just the beginning and end of the string. Perl allows us
to choose between ignoring and paying attention to newlines
by using the "//s" and "//m" modifiers. "//s" and
"//m" stand for single line and multi-line and they determine
whether a string is to be treated as one continuous
string, or as a set of lines. The two modifiers affect
two aspects of how the regexp is interpreted: 1) how the
'.' character class is defined, and 2) where the anchors
"^" and "$" are able to match. Here are the four possible
combinations:
o no modifiers (//): Default behavior. '.' matches any
character except "0. "^" matches only at the beginning
of the string and "$" matches only at the end or
before a newline at the end.
o s modifier (//s): Treat string as a single long line.
'.' matches any character, even "0. "^" matches
only at the beginning of the string and "$" matches
only at the end or before a newline at the end.
o m modifier (//m): Treat string as a set of multiple
lines. '.' matches any character except "0. "^"
and "$" are able to match at the start or end of any
line within the string.
o both s and m modifiers (//sm): Treat string as a single
long line, but detect multiple lines. '.' matches
any character, even "0. "^" and "$", however, are
able to match at the start or end of any line within
the string.
Here are examples of "//s" and "//m" in action:
$x = "There once was a girl0ho programmed in Perl0;
$x =~ /^Who/; # doesn't match, "Who" not at start of
string
$x =~ /^Who/s; # doesn't match, "Who" not at start of
string
$x =~ /^Who/m; # matches, "Who" at start of second
line
$x =~ /^Who/sm; # matches, "Who" at start of second
line
$x =~ /girl.Who/; # doesn't match, "." doesn't match
"0
$x =~ /girl.Who/s; # matches, "." matches "0
$x =~ /girl.Who/m; # doesn't match, "." doesn't match
"0
$x =~ /girl.Who/sm; # matches, "." matches "0
Most of the time, the default behavior is what is want,
but "//s" and "//m" are occasionally very useful. If
"//m" is being used, the start of the string can still be
matched with "0 matched with the anchors " (matchtheboth the end and
newline before, like "$"), and ""(matches only the
end):
$x =~ /^Who/m; # matches, "Who" at start of second
line
$x =~ /0ho" is not at start of string
$x =~ /girl$/m; # matches, "girl" at end of first
line
$x =~ /girlm; # doesn't match, "girl" is not at end of string
$x =~ /Perlm; # matche$x =~e/Perl/m;a# doesn't match, end
"Perl" is not at end of string
We now know how to create choices among classes of characters
in a regexp. What about choices among words or character
strings? Such choices are described in the next section.
Matching this or that [Toc] [Back]
Sometimes we would like to our regexp to be able to match
different possible words or character strings. This is
accomplished by using the alternation metacharacter "|".
To match "dog" or "cat", we form the regexp "dog|cat". As
before, perl will try to match the regexp at the earliest
possible point in the string. At each character position,
perl will first try to match the first alternative, "dog".
If "dog" doesn't match, perl will then try the next alternative,
"cat". If "cat" doesn't match either, then the
match fails and perl moves to the next position in the
string. Some examples:
"cats and dogs" =~ /cat|dog|bird/; # matches "cat"
"cats and dogs" =~ /dog|cat|bird/; # matches "cat"
Even though "dog" is the first alternative in the second
regexp, "cat" is able to match earlier in the string.
"cats" =~ /c|ca|cat|cats/; # matches "c"
"cats" =~ /cats|cat|ca|c/; # matches "cats"
Here, all the alternatives match at the first string position,
so the first alternative is the one that matches.
If some of the alternatives are truncations of the others,
put the longest ones first to give them a chance to match.
"cab" =~ /a|b|c/ # matches "c"
# /a|b|c/ == /[abc]/
The last example points out that character classes are
like alternations of characters. At a given character
position, the first alternative that allows the regexp
match to succeed will be the one that matches.
Grouping things and hierarchical matching [Toc] [Back]
Alternation allows a regexp to choose among alternatives,
but by itself it unsatisfying. The reason is that each
alternative is a whole regexp, but sometime we want alternatives
for just part of a regexp. For instance, suppose
we want to search for housecats or housekeepers. The regexp
"housecat|housekeeper" fits the bill, but is inefficient
because we had to type "house" twice. It would be
nice to have parts of the regexp be constant, like
"house", and some parts have alternatives, like
"cat|keeper".
The grouping metacharacters "()" solve this problem.
Grouping allows parts of a regexp to be treated as a single
unit. Parts of a regexp are grouped by enclosing them
in parentheses. Thus we could solve the "housecat|housekeeper"
by forming the regexp as "house(cat|keeper)". The
regexp "house(cat|keeper)" means match "house" followed by
either "cat" or "keeper". Some more examples are
/(a|b)b/; # matches 'ab' or 'bb'
/(ac|b)b/; # matches 'acb' or 'bb'
/(^a|b)c/; # matches 'ac' at start of string or 'bc'
anywhere
/(a|[bc])d/; # matches 'ad', 'bd', or 'cd'
/house(cat|)/; # matches either 'housecat' or 'house'
/house(cat(s|)|)/; # matches either 'housecats' or
'housecat' or
# 'house'. Note groups can be
nested.
/(19|20|)/; # match years 19xx, 20xx, or the Y2K
problem, xx
"20" =~ /(19|20|)/; # matches the null alternative
'()',
# because '20' can't match
Alternations behave the same way in groups as out of them:
at a given string position, the leftmost alternative that
allows the regexp to match is taken. So in the last example
at the first string position, "20" matches the second
alternative, but there is nothing left over to match the
next two digits "". So perl moves on to the next
alternative, which is the null alternative and that works,
since "20" is two digits.
The process of trying one alternative, seeing if it
matches, and moving on to the next alternative if it
doesn't, is called backtracking. The term 'backtracking'
comes from the idea that matching a regexp is like a walk
in the woods. Successfully matching a regexp is like
arriving at a destination. There are many possible trailheads,
one for each string position, and each one is tried
in order, left to right. From each trailhead there may be
many paths, some of which get you there, and some which
are dead ends. When you walk along a trail and hit a dead
end, you have to backtrack along the trail to an earlier
point to try another trail. If you hit your destination,
you stop immediately and forget about trying all the other
trails. You are persistent, and only if you have tried
all the trails from all the trailheads and not arrived at
your destination, do you declare failure. To be concrete,
here is a step-by-step analysis of what perl does when it
tries to match the regexp
"abcde" =~ /(abd|abc)(df|d|de)/;
0 Start with the first letter in the string 'a'.
1 Try the first alternative in the first group 'abd'.
2 Match 'a' followed by 'b'. So far so good.
3 'd' in the regexp doesn't match 'c' in the string - a
dead end. So backtrack two characters and pick the
second alternative in the first group 'abc'.
4 Match 'a' followed by 'b' followed by 'c'. We are on
a roll and have satisfied the first group. Set $1 to
'abc'.
5 Move on to the second group and pick the first alternative
'df'.
6 Match the 'd'.
7 'f' in the regexp doesn't match 'e' in the string, so
a dead end. Backtrack one character and pick the second
alternative in the second group 'd'.
8 'd' matches. The second grouping is satisfied, so set
$2 to 'd'.
9 We are at the end of the regexp, so we are done! We
have matched 'abcd' out of the string "abcde".
There are a couple of things to note about this analysis.
First, the third alternative in the second group 'de' also
allows a match, but we stopped before we got to it - at a
given character position, leftmost wins. Second, we were
able to get a match at the first character position of the
string 'a'. If there were no matches at the first position,
perl would move to the second character position 'b'
and attempt the match all over again. Only when all possible
paths at all possible character positions have been
exhausted does perl give up and declare
"$string =~ /(abd|abc)(df|d|de)/;" to be false.
Even with all this work, regexp matching happens remarkably
fast. To speed things up, during compilation stage,
perl compiles the regexp into a compact sequence of
opcodes that can often fit inside a processor cache. When
the code is executed, these opcodes can then run at full
throttle and search very quickly.
Extracting matches [Toc] [Back]
The grouping metacharacters "()" also serve another completely
different function: they allow the extraction of
the parts of a string that matched. This is very useful
to find out what matched and for text processing in
general. For each grouping, the part that matched inside
goes into the special variables $1, $2, etc. They can be
used just as ordinary variables:
# extract hours, minutes, seconds
if ($time =~ /():():()/) { # match hh:mm:ss format
$hours = $1;
$minutes = $2;
$seconds = $3;
}
Now, we know that in scalar context,
"$time =~ /():():()/" returns a true or false
value. In list context, however, it returns the list of
matched values "($1,$2,$3)". So we could write the code
more compactly as
# extract hours, minutes, seconds
($hours, $minutes, $second) = ($time =~ /():():()/);
If the groupings in a regexp are nested, $1 gets the group
with the leftmost opening parenthesis, $2 the next opening
parenthesis, etc. For example, here is a complex regexp
and the matching variables indicated below it:
/(ab(cd|ef)((gi)|j))/;
1 2 34
so that if the regexp matched, e.g., $2 would contain 'cd'
or 'ef'. For convenience, perl sets $+ to the string held
by the highest numbered $1, $2, ... that got assigned
(and, somewhat related, $^N to the value of the $1, $2,
... most-recently assigned; i.e. the $1, $2, ... associated
with the rightmost closing parenthesis used in the
match).
Closely associated with the matching variables $1, $2, ...
are the backreferences "1", "2", ... . Backreferences
are simply matching variables that can be used inside a
regexp. This is a really nice feature - what matches
later in a regexp can depend on what matched earlier in
the regexp. Suppose we wanted to look for doubled words
in text, like 'the the'. The following regexp finds all
3-letter doubles with a space in between:
/(24
The grouping assigns a value to 1, so that the same 3
letter sequence is used for both parts. Here are some
words with repeated parts:
% simple_grep '^(24
beriberi
booboo
coco
mama
murmur
papa
The regexp has a single grouping which considers 4-letter
combinations, then 3-letter combinations, etc. and uses
"1" to look for a repeat. Although $1 and "1" represent
the same thing, care should be taken to use matched variables
$1, $2, ... only outside a regexp and backreferences
"1", "2", ... only inside a regexp; not doing so may
lead to surprising and/or undefined results.
In addition to what was matched, Perl 5.6.0 also provides
the positions of what was matched with the "@-" and "@+"
arrays. "$-[0]" is the position of the start of the entire
match and $+[0] is the position of the end. Similarly,
"$-[n]" is the position of the start of the $n match and
$+[n] is the position of the end. If $n is undefined, so
are "$-[n]" and $+[n]. Then this code
$x = "Mmm...donut, thought Homer";
$x =~ /^(Mmm|Yech)...(donut|peas)/; # matches
foreach $expr (1..$#-) {
print "Match $expr: '${$expr}' at position
($-[$expr],$+[$expr])0;
}
prints
Match 1: 'Mmm' at position (0,3)
Match 2: 'donut' at position (6,11)
Even if there are no groupings in a regexp, it is still
possible to find out what exactly matched in a string. If
you use them, perl will set $` to the part of the string
before the match, will set $& to the part of the string
that matched, and will set $' to the part of the string
after the match. An example:
$x = "the cat caught the mouse";
$x =~ /cat/; # $` = 'the ', $& = 'cat', $' = ' caught
the mouse'
$x =~ /the/; # $` = '', $& = 'the', $' = ' cat caught
the mouse'
In the second match, "$` = ''" because the regexp matched
at the first character position in the string and stopped,
it never saw the second 'the'. It is important to note
that using $` and $' slows down regexp matching quite a
bit, and $& slows it down to a lesser extent, because if
they are used in one regexp in a program, they are generated
for <all> regexps in the program. So if raw performance
is a goal of your application, they should be
avoided. If you need them, use "@-" and "@+" instead:
$` is the same as substr( $x, 0, $-[0] )
$& is the same as substr( $x, $-[0], $+[0]-$-[0] )
$' is the same as substr( $x, $+[0] )
Matching repetitions [Toc] [Back]
The examples in the previous section display an annoying
weakness. We were only matching 3-letter words, or syllables
of 4 letters or less. We'd like to be able to match
words or syllables of any length, without writing out
tedious alternatives like "24
This is exactly the problem the quantifier metacharacters
"?", "*", "+", and "{}" were created for. They allow us
to determine the number of repeats of a portion of a regexp
we consider to be a match. Quantifiers are put immediately
after the character, character class, or grouping
that we want to specify. They have the following meanings:
o "a?" = match 'a' 1 or 0 times
o "a*" = match 'a' 0 or more times, i.e., any number of
times
o "a+" = match 'a' 1 or more times, i.e., at least once
o "a{n,m}" = match at least "n" times, but not more than
"m" times.
o "a{n,}" = match at least "n" or more times
o "a{n}" = match exactly "n" times
Here are some examples:
/[a-z]+*/; # match a lowercase word, at least some
space, and
# any number of digits
/(1152
/y(es)?/i; # matches 'y', 'Y', or a case-insensitive 'yes'
$year =~ /{2,4}/; # make sure year is at least 2 but
not more
# than 4 digits
$year =~ /{4}|{2}/; # better match; throw out 3
digit dates
$year =~ /{2}({2})?/; # same thing written differently. However,
# this produces $1 and the
other does not.
% simple_grep '^(1032
beriberi
booboo
coco
mama
murmur
papa
For all of these quantifiers, perl will try to match as
much of the string as possible, while still allowing the
regexp to succeed. Thus with "/a?.../", perl will first
try to match the regexp with the "a" present; if that
fails, perl will try to match the regexp without the "a"
present. For the quantifier "*", we get the following:
$x = "the cat in the hat";
$x =~ /^(.*)(cat)(.*)$/; # matches,
# $1 = 'the '
# $2 = 'cat'
# $3 = ' in the hat'
Which is what we might expect, the match finds the only
"cat" in the string and locks onto it. Consider, however,
this regexp:
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)
One might initially guess that perl would find the "at" in
"cat" and stop there, but that wouldn't give the longest
possible string to the first quantifier ".*". Instead,
the first quantifier ".*" grabs as much of the string as
possible while still having the regexp match. In this
example, that means having the "at" sequence with the
final "at" in the string. The other important principle
illustrated here is that when there are two or more elements
in a regexp, the leftmost quantifier, if there is
one, gets to grab as much the string as possible, leaving
the rest of the regexp to fight over scraps. Thus in our
example, the first quantifier ".*" grabs most of the
string, while the second quantifier ".*" gets the empty
string. Quantifiers that grab as much of the string as
possible are called maximal match or greedy quantifiers.
When a regexp can match a string in several different
ways, we can use the principles above to predict which way
the regexp will match:
o Principle 0: Taken as a whole, any regexp will be
matched at the earliest possible position in the
string.
o Principle 1: In an alternation "a|b|c...", the leftmost
alternative that allows a match for the whole
regexp will be the one used.
o Principle 2: The maximal matching quantifiers "?",
"*", "+" and "{n,m}" will in general match as much of
the string as possible while still allowing the whole
regexp to match.
o Principle 3: If there are two or more elements in a
regexp, the leftmost greedy quantifier, if any, will
match as much of the string as possible while still
allowing the whole regexp to match. The next leftmost
greedy quantifier, if any, will try to match as much
of the string remaining available to it as possible,
while still allowing the whole regexp to match. And
so on, until all the regexp elements are satisfied.
As we have seen above, Principle 0 overrides the others -
the regexp will be matched as early as possible, with the
other principles determining how the regexp matches at
that earliest character position.
Here is an example of these principles in action:
$x = "The programming republic of Perl";
$x =~ /^(.+)(e|r)(.*)$/; # matches,
# $1 = 'The programming republic of Pe'
# $2 = 'r'
# $3 = 'l'
This regexp matches at the earliest string position, 'T'.
One might think that "e", being leftmost in the alternation,
would be matched, but "r" produces the longest
string in the first quantifier.
$x =~ /(m{1,2})(.*)$/; # matches,
# $1 = 'mm'
# $2 = 'ing republic of Perl'
Here, The earliest possible match is at the first 'm' in
"programming". "m{1,2}" is the first quantifier, so it
gets to match a maximal "mm".
$x =~ /.*(m{1,2})(.*)$/; # matches,
# $1 = 'm'
# $2 = 'ing republic of
Perl'
Here, the regexp matches at the start of the string. The
first quantifier ".*" grabs as much as possible, leaving
just a single 'm' for the second quantifier "m{1,2}".
$x =~ /(.?)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of
Perl'
Here, ".?" eats its maximal one character at the earliest
possible position in the string, 'a' in "programming",
leaving "m{1,2}" the opportunity to match both "m"'s.
Finally,
"aXXXb" =~ /(X*)/; # matches with $1 = ''
because it can match zero copies of 'X' at the beginning
of the string. If you definitely want to match at least
one 'X', use "X+", not "X*".
Sometimes greed is not good. At times, we would like
quantifiers to match a minimal piece of string, rather
than a maximal piece. For this purpose, Larry Wall created
the minimal match or non-greedy quantifiers
"??","*?", "+?", and "{}?". These are the usual quantifiers
with a "?" appended to them. They have the following
meanings:
o "a??" = match 'a' 0 or 1 times. Try 0 first, then 1.
o "a*?" = match 'a' 0 or more times, i.e., any number of
times, but as few times as possible
o "a+?" = match 'a' 1 or more times, i.e., at least
once, but as few times as possible
o "a{n,m}?" = match at least "n" times, not more than
"m" times, as few times as possible
o "a{n,}?" = match at least "n" times, but as few times
as possible
o "a{n}?" = match exactly "n" times. Because we match
exactly "n" times, "a{n}?" is equivalent to "a{n}" and
is just there for notational consistency.
Let's look at the example above, but with minimal quantifiers:
$x = "The programming republic of Perl";
$x =~ /^(.+?)(e|r)(.*)$/; # matches,
# $1 = 'Th'
# $2 = 'e'
# $3 = ' programming republic of Perl'
The minimal string that will allow both the start of the
string "^" and the alternation to match is "Th", with the
alternation "e|r" matching "e". The second quantifier
".*" is free to gobble up the rest of the string.
$x =~ /(m{1,2}?)(.*?)$/; # matches,
# $1 = 'm'
# $2 = 'ming republic of
Perl'
The first string position that this regexp can match is at
the first 'm' in "programming". At this position, the minimal
"m{1,2}?" matches just one 'm'. Although the second
quantifier ".*?" would prefer to match no characters, it
is constrained by the end-of-string anchor "$" to match
the rest of the string.
$x =~ /(.*?)(m{1,2}?)(.*)$/; # matches,
# $1 = 'The progra'
# $2 = 'm'
# $3 = 'ming republic of
Perl'
In this regexp, you might expect the first minimal quantifier
".*?" to match the empty string, because it is not
constrained by a "^" anchor to match the beginning of the
word. Principle 0 applies here, however. Because it is
possible for the whole regexp to match at the start of the
string, it will match at the start of the string. Thus
the first quantifier has to match everything up to the
first "m". The second minimal quantifier matches just one
"m" and the third quantifier matches the rest of the
string.
$x =~ /(.??)(m{1,2})(.*)$/; # matches,
# $1 = 'a'
# $2 = 'mm'
# $3 = 'ing republic of
Perl'
Just as in the previous regexp, the first quantifier ".??"
can match earliest at position 'a', so it does. The second
quantifier is greedy, so it matches "mm", and the
third matches the rest of the string.
We can modify principle 3 above to take into account nongreedy
quantifiers:
o Principle 3: If there are two or more elements in a
regexp, the leftmost greedy (non-greedy) quantifier,
if any, will match as much (little) of the string as
possible while still allowing the whole regexp to
match. The next leftmost greedy (non-greedy) quantifier,
if any, will try to match as much (little) of
the string remaining available to it as possible,
while still allowing the whole regexp to match. And
so on, until all the regexp elements are satisfied.
Just like alternation, quantifiers are also susceptible to
backtracking. Here is a step-by-step analysis of the
example
$x = "the cat in the hat";
$x =~ /^(.*)(at)(.*)$/; # matches,
# $1 = 'the cat in the h'
# $2 = 'at'
# $3 = '' (0 matches)
0 Start with the first letter in the string 't'.
1 The first quantifier '.*' starts out by matching the
whole string 'the cat in the hat'.
2 'a' in the regexp element 'at' doesn't match the end
of the string. Backtrack one character.
3 'a' in the regexp element 'at' still doesn't match the
last letter of the string 't', so backtrack one more
character.
4 Now we can match the 'a' and the 't'.
5 Move on to the third element '.*'. Since we are at
the end of the string and '.*' can match 0 times,
assign it the empty string.
6 We are done!
Most of the time, all this moving forward and backtracking
happens quickly and searching is fast. There are some
pathological regexps, however, whose execution time exponentially
grows with the size of the string. A typical
structure that blows up in your face is of the form
/(a|b+)*/;
The problem is the nested indeterminate quantifiers.
There are many different ways of partitioning a string of
length n between the "+" and "*": one repetition with "b+"
of length n, two repetitions with the first "b+" length k
and the second with length n-k, m repetitions whose bits
add up to length n, etc. In fact there are an exponential
number of ways to partition a string as a function of
length. A regexp may get lucky and match early in the
process, but if there is no match, perl will try every
possibility before giving up. So be careful with nested
"*"'s, "{n,m}"'s, and "+"'s. The book Mastering regular
expressions by Jeffrey Friedl gives a wonderful discussion
of this and other efficiency issues.
Building a regexp [Toc] [Back]
At this point, we have all the basic regexp concepts covered,
so let's give a more involved example of a regular
expression. We will build a regexp that matches numbers.
The first task in building a regexp is to decide what we
want to match and what we want to exclude. In our case,
we want to match both integers and floating point numbers
and we want to reject any string that isn't a number.
The next task is to break the problem down into smaller
problems that are easily converted into a regexp.
The simplest case is integers. These consist of a
sequence of digits, with an optional sign in front. The
digits we can represent with "+" and the sign can be
matched with "[+-]". Thus the integer regexp is
/[+-]?+/; # matches integers
A floating point number potentially has a sign, an integral
part, a decimal point, a fractional part, and an
exponent. One or more of these parts is optional, so we
need to check out the different possibilities. Floating
point numbers which are in proper form include 123.,
0.345, .34, -1e6, and 25.4E-72. As with integers, the
sign out front is completely optional and can be matched
by "[+-]?". We can see that if there is no exponent,
floating point numbers must have a decimal point, otherwise
they are integers. We might be tempted to model
these with "*.*", but this would also match just a
single decimal point, which is not a number. So the three
cases of floating point number sans exponent are
/[+-]?+./; # 1., 321., etc.
/[+-]?.+/; # .1, .234, etc.
/[+-]?+.+/; # 1.0, 30.56, etc.
These can be combined into a single regexp with a threeway
alternation:
/[+-]?(+.+|+.|.+)/; # floating point, no exponent
In this alternation, it is important to put '+.+'
before '+.'. If '+.' were first, the regexp would
happily match that and ignore the fractional part of the
number.
Now consider floating point numbers with exponents. The
key observation here is that both integers and numbers
with decimal points are allowed in front of an exponent.
Then exponents, like the overall sign, are independent of
whether we are matching numbers with or without decimal
points, and can be 'decoupled' from the mantissa. The
overall form of the regexp now becomes clear:
/^(optional sign)(integer | f.p. mantissa)(optional
exponent)$/;
The exponent is an "e" or "E", followed by an integer. So
the exponent regexp is
/[eE][+-]?+/; # exponent
Putting all the parts together, we get a regexp that
matches numbers:
/^[+-]?(+.+|+.|.+|+)([eE][+-]?+)?$/; # Ta da!
Long regexps like this may impress your friends, but can
be hard to decipher. In complex situations like this, the
"//x" modifier for a match is invaluable. It allows one
to put nearly arbitrary whitespace and comments into a
regexp without affecting their meaning. Using it, we can
rewrite our 'extended' regexp in the more pleasing form
/^
[+-]? # first, match an optional sign
( # then match integers or f.p. mantissas:
+.+ # mantissa of the form a.b
|+. # mantissa of the form a.
|.+ # mantissa of the form .b
|+ # integer of the form a
)
([eE][+-]?+)? # finally, optionally match an exponent
$/x;
If whitespace is mostly irrelevant, how does one include
space characters in an extended regexp? The answer is to
backslash it ' ' or put it in a character class "[ ]" .
The same thing goes for pound signs, use " For instance, Perl allows a space between the sign and the
mantissa/integer, and we could add this to our regexp as
follows:
/^
[+-]? * # first, match an optional sign *and
space*
( # then match integers or f.p. mantissas:
+.+ # mantissa of the form a.b
|+. # mantissa of the form a.
|.+ # mantissa of the form .b
|+ # integer of the form a
)
([eE][+-]?+)? # finally, optionally match an exponent
$/x;
In this form, it is easier to see a way to simplify the
alternation. Alternatives 1, 2, and 4 all start with
"+", so it could be factored out:
/^
[+-]? * # first, match an optional sign
( # then match integers or f.p. mantissas:
+ # start out with a ...
(
.* # mantissa of the form a.b or a.
)? # ? takes care of integers of the form
a
|.+ # mantissa of the form .b
)
([eE][+-]?+)? # finally, optionally match an exponent
$/x;
or written in the compact form,
/^[+-]? *(+(.*)?|.+)([eE][+-]?+)?$/;
This is our final regexp. To recap, we built a regexp by
o specifying the task in detail,
o breaking down the problem into smaller parts,
o translating the small parts into regexps,
o combining the regexps,
o and optimizing the final combined regexp.
These are also the typical steps involved in writing a
computer program. This makes perfect sense, because regular
expressions are essentially programs written a little
computer language that specifies patterns.
Using regular expressions in Perl [Toc] [Back]
The last topic of Part 1 briefly covers how regexps are
used in Perl programs. Where do they fit into Perl syntax?
We have already introduced the matching operator in its
default "/regexp/" and arbitrary delimiter "m!regexp!"
forms. We have used the binding operator "=~" and its
negation "!~" to test for string matches. Associated with
the matching operator, we have discussed the single line
"//s", multi-line "//m", case-insensitive "//i" and
extended "//x" modifiers.
There are a few more things you might want to know about
matching operators. First, we pointed out earlier that
variables in regexps are substituted before the regexp is
evaluated:
$pattern = 'Seuss';
while (<>) {
print if /$pattern/;
}
This will print any lines containing the word "Seuss". It
is not as efficient as it could be, however, because perl
has to re-evaluate $pattern each time through the loop.
If $pattern won't be changing over the lifetime of the
script, we can add the "//o" modifier, which directs perl
to only perform variable substitutions once:
#!/usr/bin/perl
# Improved simple_grep
$regexp = shift;
while (<>) {
print if /$regexp/o; # a good deal faster
}
If you change $pattern after the first substitution happens,
perl will ignore it. If you don't want any substitutions
at all, use the special delimiter "m''":
@pattern = ('Seuss');
while (<>) {
print if m'@pattern'; # matches literal '@pattern', not 'Seuss'
}
"m''" acts like single quotes on a regexp; all other "m"
delimiters act like double quotes. If the regexp evaluates
to the empty string, the regexp in the last success-
ful match is used instead. So we have
"dog" =~ /d/; # 'd' matches
"dogbert =~ //; # this matches the 'd' regexp used
before
The final two modifiers "//g" and "//c" concern multiple
matches. The modifier "//g" stands for global matching
and allows the matching operator to match within a string
as many times as possible. In scalar context, successive
invocations against a string will have `"//g" jump from
match to match, keeping track of position in the string as
it goes along. You can get or set the position with the
"pos()" function.
The use of "//g" is shown in the following example. Suppose
we have a string that consists of words separated by
spaces. If we know how many words there are in advance,
we could extract the words using groupings:
$x = "cat dog house"; # 3 words
$x =~ /^(24
# $1 = 'cat'
# $2 = 'dog'
# $3 = 'house'
But what if we had an indeterminate number of words? This
is the sort of task "//g" was made for. To extract all
words, form the simple regexp "(480
matches with "/(120
while ($x =~ /(144
print "Word is $1, ends at position ", pos $x, "0;
}
prints
Word is cat, ends at position 3
Word is dog, ends at position 7
Word is house, ends at position 13
A failed match or changing the target string resets the
position. If you don't want the position reset after
|
