X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/353c650532037e4006fbdb2176350717f320f7c3..fb5f378b17e3b41db03064c19b9205db64a3354c:/pod/perlretut.pod diff --git a/pod/perlretut.pod b/pod/perlretut.pod index 67e0670..1e1cdd4 100644 --- a/pod/perlretut.pod +++ b/pod/perlretut.pod @@ -17,21 +17,42 @@ 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; +What is a regular expression? At its most basic, a regular expression +is a template that is used to determine if a string has certain +characteristics. The string is most often some text, such as a line, +sentence, web page, or even a whole book, but less commonly it could be +some binary data as well. +Suppose we want to determine if the text in variable, C<$var> contains +the sequence of characters S> +(blanks added for legibility). We can write in Perl + + $var =~ m/mushroom/ + +The value of this expression will be TRUE if C<$var> contains that +sequence of characters, and FALSE otherwise. The portion enclosed in +C<'E'> characters denotes the characteristic we are looking for. +We use the term I for it. The process of looking to see if the +pattern occurs in the string is called I, and the C<"=~"> +operator along with the C tell Perl to try to match the pattern +against the string. Note that the pattern is also a string, but a very +special kind of one, as we will see. 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., C -or C. 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. +and the patterns used to list files in a directory, I, "C" +or "C". In Perl, the patterns described by regular expressions +are used not only to search strings, but to also 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 +and difficult to understand. This really stems simply because the +notation used to express them tends to be terse and dense, and not +because of inherent complexity. We recommend using the C regular +expression modifier (described below) along with plenty of white space +to make them less dense, and easier to read. Regular expressions are +constructed using simple concepts like conditionals and loops and are no more difficult to understand than the corresponding C conditionals and C -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. +loops in the Perl language itself. This tutorial flattens the learning curve by discussing regular expression concepts, along with their notation, one at a time and with @@ -41,26 +62,30 @@ 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. +introduces the latest cutting-edge innovations. -A note: to save time, 'regular expression' is often abbreviated as +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. +New in v5.22, L|re/'strict' mode> applies stricter +rules than otherwise when compiling regular expression patterns. It can +find things that, while legal, may not be what you intended. + =head1 Part 1: The basics =head2 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 +characters. A regexp consisting of just a word matches any string that contains that word: "Hello World" =~ /World/; # matches What is this Perl statement all about? C<"Hello World"> is a simple -double quoted string. C is the regular expression and the +double-quoted string. C is the regular expression and the C enclosing C tells Perl to search a string for a match. The operator C<=~> associates the string with the regexp match and produces a true value if the regexp matched, or false if the regexp @@ -87,7 +112,7 @@ be reversed by using the C operator: The literal string in the regexp can be replaced by a variable: - $greeting = "World"; + my $greeting = "World"; if ("Hello World" =~ /$greeting/) { print "It matches\n"; } @@ -115,7 +140,7 @@ to arbitrary delimiters by putting an C<'m'> out front: # '/' becomes an ordinary char C, C, and C all represent the -same thing. When, e.g., the quote (C<">) is used as a delimiter, the forward +same thing. When, I, the quote (C<'"'>) is used as a delimiter, the forward slash C<'/'> becomes an ordinary character and can be used in this regexp without trouble. @@ -129,10 +154,10 @@ Let's consider how different regexps would match C<"Hello World">: The first regexp C doesn't match because regexps are case-sensitive. The second regexp matches because the substring S> occurs in the string S>. The space -character ' ' is treated like any other character in a regexp and is +character C<' '> 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 C<'oW'> doesn't match. The fourth regexp -C<'World '> doesn't match because there is a space at the end of the +"C" 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 I in order for the statement to be true. @@ -144,15 +169,22 @@ always match at the earliest possible point in the string: "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 I, are reserved -for use in regexp notation. The metacharacters are +need to know about. First of all, not all characters can be used "as +is" in a match. Some characters, called I, are +generally reserved for use in regexp notation. The metacharacters are + + {}[]()^$.|*+?-#\ - {}[]()^$.|*+?\ +This list is not as definitive as it may appear (or be claimed to be in +other documentation). For example, C<"#"> is a metacharacter only when +the C pattern modifier (described below) is used, and both C<"}"> +and C<"]"> are metacharacters only when paired with opening C<"{"> or +C<"["> respectively; other gotchas apply. 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: +that a metacharacter can be matched as-is 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 + @@ -172,19 +204,28 @@ be backslashed: 'C:\WIN32' =~ /C:\\WIN/; # matches +In situations where it doesn't make sense for a particular metacharacter +to mean what it normally does, it automatically loses its +metacharacter-ness and becomes an ordinary character that is to be +matched literally. For example, the C<'}'> is a metacharacter only when +it is the mate of a C<'{'> metacharacter. Otherwise it is treated as a +literal RIGHT CURLY BRACKET. This may lead to unexpected results. +L|re/'strict' mode> can catch some of these. + In addition to the metacharacters, there are some ASCII characters which don't have printable character equivalents and are instead represented by I. Common examples are C<\t> for a tab, C<\n> for a newline, C<\r> for a carriage return and C<\a> for a -bell. If your string is better thought of as a sequence of arbitrary -bytes, the octal escape sequence, e.g., C<\033>, or hexadecimal escape -sequence, e.g., C<\x1B> may be a more natural representation for your +bell (or alert). If your string is better thought of as a sequence of arbitrary +bytes, the octal escape sequence, I, C<\033>, or hexadecimal escape +sequence, I, C<\x1B> may be a more natural representation for your bytes. Here are some examples of escapes: "1000\t2000" =~ m(0\t2) # matches "1000\n2000" =~ /0\n20/ # matches "1000\t2000" =~ /\000\t2/ # doesn't match, "0" ne "\000" - "cat" =~ /\143\x61\x74/ # matches, but a weird way to spell cat + "cat" =~ /\o{143}\x61\x74/ # matches in ASCII, 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 @@ -236,9 +277,9 @@ C use the default variable C<$_> implicitly. With all of the regexps above, if the regexp matched anywhere in the string, it was considered a match. Sometimes, however, we'd like to specify I in the string the regexp should try to match. To do -this, we would use the I metacharacters C<^> and C<$>. The -anchor C<^> means match at the beginning of the string and the anchor -C<$> means match at the end of the string, or before a newline at the +this, we would use the I metacharacters C<'^'> and C<'$'>. The +anchor C<'^'> means match at the beginning of the string and the anchor +C<'$'> means match at the end of the string, or before a newline at the end of the string. Here is how they are used: "housekeeper" =~ /keeper/; # matches @@ -246,13 +287,13 @@ end of the string. Here is how they are used: "housekeeper" =~ /keeper$/; # matches "housekeeper\n" =~ /keeper$/; # matches -The second regexp doesn't match because C<^> constrains C to +The second regexp doesn't match because C<'^'> constrains C to match only at the beginning of the string, but C<"housekeeper"> has keeper starting in the middle. The third regexp does match, since the -C<$> constrains C to match only at the end of the string. +C<'$'> constrains C to match only at the end of the string. -When both C<^> and C<$> are used at the same time, the regexp has to -match both the beginning and the end of the string, i.e., the regexp +When both C<'^'> and C<'$'> are used at the same time, the regexp has to +match both the beginning and the end of the string, I, the regexp matches the whole string. Consider "keeper" =~ /^keep$/; # doesn't match @@ -261,7 +302,7 @@ matches the whole string. Consider The first regexp doesn't match because the string has more to it than C. Since the second regexp is exactly the string, it -matches. Using both C<^> and C<$> in a regexp forces the complete +matches. Using both C<'^'> and C<'$'> 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: @@ -286,13 +327,14 @@ 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 I of them. One such concept is that of a I. 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 C<[...]>, with the set of characters +character, to match at a particular point in a regexp. You can define +your own custom character classes. These +are denoted by brackets C<[...]>, with the set of characters to be possibly matched inside. Here are some examples: /cat/; # matches 'cat' @@ -316,13 +358,13 @@ 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 C<\> to represent themselves. The same is true in a +backslash C<'\'> to represent themselves. 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 C<-]\^$> (and the pattern delimiter, whatever it is). -C<]> is special because it denotes the end of a character class. C<$> is -special because it denotes a scalar variable. C<\> is special because +C<']'> is special because it denotes the end of a character class. C<'$'> is +special because it denotes a scalar variable. C<'\'> is special because it is used in escape sequences, just like above. Here is how the special characters C<]$\> are handled: @@ -333,7 +375,7 @@ special characters C<]$\> are handled: /[\\$x]at/; # matches '\at', 'bat, 'cat', or 'rat' The last two are a little tricky. In C<[\$x]>, the backslash protects -the dollar sign, so the character class has two members C<$> and C. +the dollar sign, so the character class has two members C<'$'> and C<'x'>. In C<[\\$x]>, the backslash is protected, so C<$x> is treated as a variable and substituted in double quote fashion. @@ -353,7 +395,7 @@ If C<'-'> is the first or last character in a character class, it is treated as an ordinary character; C<[-ab]>, C<[ab-]> and C<[a\-b]> are all equivalent. -The special character C<^> in the first position of a character class +The special character C<'^'> in the first position of a character class denotes a I, which matches any character but those in the brackets. Both C<[...]> and C<[^...]> must match a character, or the match fails. Then @@ -366,45 +408,60 @@ character, or the match fails. Then Now, even C<[0-9]> can be a bother to write multiple times, so in the interest of saving keystrokes and making regexps more readable, Perl has several abbreviations for common character classes, as shown below. -Since the introduction of Unicode, these character classes match more -than just a few characters in the ISO 8859-1 range. +Since the introduction of Unicode, unless the C modifier is in +effect, these character classes match more than just a few characters in +the ASCII range. =over 4 =item * -\d matches a digit, not just [0-9] but also digits from non-roman scripts +C<\d> matches a digit, not just C<[0-9]> but also digits from non-roman scripts =item * -\s matches a whitespace character, the set [\ \t\r\n\f] and others +C<\s> matches a whitespace character, the set C<[\ \t\r\n\f]> and others =item * -\w matches a word character (alphanumeric or _), not just [0-9a-zA-Z_] +C<\w> matches a word character (alphanumeric or C<'_'>), not just C<[0-9a-zA-Z_]> but also digits and characters from non-roman scripts =item * -\D is a negated \d; it represents any other character than a digit, or [^\d] +C<\D> is a negated C<\d>; it represents any other character than a digit, or C<[^\d]> =item * -\S is a negated \s; it represents any non-whitespace character [^\s] +C<\S> is a negated C<\s>; it represents any non-whitespace character C<[^\s]> =item * -\W is a negated \w; it represents any non-word character [^\w] +C<\W> is a negated C<\w>; it represents any non-word character C<[^\w]> =item * -The period '.' matches any character but "\n" (unless the modifier C is +The period C<'.'> matches any character but C<"\n"> (unless the modifier C is in effect, as explained below). +=item * + +C<\N>, like the period, matches any character but C<"\n">, but it does so +regardless of whether the modifier C is in effect. + =back +The C modifier, available starting in Perl 5.14, is used to +restrict the matches of C<\d>, C<\s>, and C<\w> to just those in the ASCII range. +It is useful to keep your program from being needlessly exposed to full +Unicode (and its accompanying security considerations) when all you want +is to process English-like text. (The "a" may be doubled, C, to +provide even more restrictions, preventing case-insensitive matching of +ASCII with non-ASCII characters; otherwise a Unicode "Kelvin Sign" +would caselessly match a "k" or "K".) + The C<\d\s\w\D\S\W> abbreviations can be used both inside and outside -of character classes. Here are some in use: +of bracketed character classes. Here are some in use: /\d\d:\d\d:\d\d/; # matches a hh:mm:ss time format /[\d\s]/; # matches any digit or whitespace character @@ -420,6 +477,11 @@ of characters, it is incorrect to think of C<[^\d\w]> as C<[\D\W]>; in fact C<[^\d\w]> is the same as C<[^\w]>, which is the same as C<[\W]>. Think DeMorgan's laws. +In actuality, the period and C<\d\s\w\D\S\W> abbreviations are +themselves types of character classes, so the ones surrounded by +brackets are just one type of character class. When we need to make a +distinction, we refer to them as "bracketed character classes." + An anchor useful in basic regexps is the I C<\b>. This matches a boundary between a word character and a non-word character C<\w\W> or C<\W\w>: @@ -433,6 +495,11 @@ character C<\w\W> or C<\W\w>: Note in the last example, the end of the string is considered a word boundary. +For natural language processing (so that, for example, apostrophes are +included in words), use instead C<\b{wb}> + + "don't" =~ / .+? \b{wb} /x; # matches the whole string + You might wonder why C<'.'> matches everything but C<"\n"> - why not every character? The reason is that often one is matching against lines and would like to ignore the newline characters. For instance, @@ -450,48 +517,48 @@ of it as empty. Then 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 C<^> -and C<$> to anchor at the beginning and end of lines within the +however, we want to keep track of newlines. We might even want C<'^'> +and C<'$'> to anchor at the beginning and end of lines within the string, rather than just the beginning and end of the string. Perl allows us to choose between ignoring and paying attention to newlines -by using the C and C modifiers. C and C stand for +by using the C and C modifiers. C and C stand for single line and multi-line and they determine whether a string is to be treated as one continuous string, or as a set of lines. The two modifiers affect two aspects of how the regexp is interpreted: 1) how -the C<'.'> character class is defined, and 2) where the anchors C<^> -and C<$> are able to match. Here are the four possible combinations: +the C<'.'> character class is defined, and 2) where the anchors C<'^'> +and C<'$'> are able to match. Here are the four possible combinations: =over 4 =item * -no modifiers (//): Default behavior. C<'.'> matches any character -except C<"\n">. C<^> matches only at the beginning of the string and -C<$> matches only at the end or before a newline at the end. +no modifiers: Default behavior. C<'.'> matches any character +except C<"\n">. C<'^'> matches only at the beginning of the string and +C<'$'> matches only at the end or before a newline at the end. =item * -s modifier (//s): Treat string as a single long line. C<'.'> matches -any character, even C<"\n">. C<^> matches only at the beginning of -the string and C<$> matches only at the end or before a newline at the +s modifier (C): Treat string as a single long line. C<'.'> matches +any character, even C<"\n">. C<'^'> matches only at the beginning of +the string and C<'$'> matches only at the end or before a newline at the end. =item * -m modifier (//m): Treat string as a set of multiple lines. C<'.'> -matches any character except C<"\n">. C<^> and C<$> are able to match +m modifier (C): Treat string as a set of multiple lines. C<'.'> +matches any character except C<"\n">. C<'^'> and C<'$'> are able to match at the start or end of I line within the string. =item * -both s and m modifiers (//sm): Treat string as a single long line, but +both s and m modifiers (C): Treat string as a single long line, but detect multiple lines. C<'.'> matches any character, even -C<"\n">. C<^> and C<$>, however, are able to match at the start or end +C<"\n">. C<'^'> and C<'$'>, however, are able to match at the start or end of I line within the string. =back -Here are examples of C and C in action: +Here are examples of C and C in action: $x = "There once was a girl\nWho programmed in Perl\n"; @@ -505,11 +572,11 @@ Here are examples of C and C in action: $x =~ /girl.Who/m; # doesn't match, "." doesn't match "\n" $x =~ /girl.Who/sm; # matches, "." matches "\n" -Most of the time, the default behavior is what is wanted, but C and -C are occasionally very useful. If C is being used, the start +Most of the time, the default behavior is what is wanted, but C and +C are occasionally very useful. If C is being used, the start of the string can still be matched with C<\A> and the end of the string can still be matched with the anchors C<\Z> (matches both the end and -the newline before, like C<$>), and C<\z> (matches only the end): +the newline before, like C<'$'>), and C<\z> (matches only the end): $x =~ /^Who/m; # matches, "Who" at start of second line $x =~ /\AWho/m; # doesn't match, "Who" is not at start of string @@ -528,7 +595,7 @@ choices are described in the next section. Sometimes we would like our regexp to be able to match different possible words or character strings. This is accomplished by using -the I metacharacter C<|>. To match C or C, we +the I metacharacter C<'|'>. To match C or C, we form the regexp C. 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 @@ -602,7 +669,7 @@ C<"20"> is two digits. The process of trying one alternative, seeing if it matches, and moving on to the next alternative, while going back in the string from where the previous alternative was tried, if it doesn't, is called -I. The term 'backtracking' comes from the idea that +I. 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 @@ -620,62 +687,59 @@ of what Perl does when it tries to match the regexp =over 4 -=item 0 - -Start with the first letter in the string 'a'. +=item Z<>0. Start with the first letter in the string C<'a'>. -=item 1 +E -Try the first alternative in the first group 'abd'. +=item Z<>1. Try the first alternative in the first group C<'abd'>. -=item 2 +E -Match 'a' followed by 'b'. So far so good. +=item Z<>2. Match C<'a'> followed by C<'b'>. So far so good. -=item 3 +E -'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'. +=item Z<>3. C<'d'> in the regexp doesn't match C<'c'> in the string - a +dead end. So backtrack two characters and pick the second alternative +in the first group C<'abc'>. -=item 4 +E -Match 'a' followed by 'b' followed by 'c'. We are on a roll -and have satisfied the first group. Set $1 to 'abc'. +=item Z<>4. Match C<'a'> followed by C<'b'> followed by C<'c'>. We are on a roll +and have satisfied the first group. Set C<$1> to C<'abc'>. -=item 5 +E -Move on to the second group and pick the first alternative -'df'. +=item Z<>5 Move on to the second group and pick the first alternative C<'df'>. -=item 6 +E -Match the 'd'. +=item Z<>6 Match the C<'d'>. -=item 7 +E -'f' in the regexp doesn't match 'e' in the string, so a dead +=item Z<>7. C<'f'> in the regexp doesn't match C<'e'> in the string, so a dead end. Backtrack one character and pick the second alternative in the -second group 'd'. +second group C<'d'>. -=item 8 +E -'d' matches. The second grouping is satisfied, so set $2 to -'d'. +=item Z<>8. C<'d'> matches. The second grouping is satisfied, so set +C<$2> to C<'d'>. -=item 9 +E -We are at the end of the regexp, so we are done! We have -matched 'abcd' out of the string "abcde". +=item Z<>9. We are at the end of the regexp, so we are done! We have +matched C<'abcd'> out of the string C<"abcde">. =back 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 +third alternative in the second group C<'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 +position of the string C<'a'>. If there were no matches at the first +position, Perl would move to the second character position C<'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 S> to be false. @@ -692,7 +756,7 @@ The grouping metacharacters C<()> 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 C<$1>, C<$2>, etc. They can be +inside goes into the special variables C<$1>, C<$2>, I. They can be used just as ordinary variables: # extract hours, minutes, seconds @@ -712,7 +776,7 @@ C<($1,$2,$3)>. So we could write the code more compactly as If the groupings in a regexp are nested, C<$1> gets the group with the leftmost opening parenthesis, C<$2> the next opening parenthesis, -etc. Here is a regexp with nested groups: +I. Here is a regexp with nested groups: /(ab(cd|ef)((gi)|j))/; 1 2 34 @@ -724,7 +788,7 @@ or it remains undefined. For convenience, Perl sets C<$+> to the string held by the highest numbered C<$1>, C<$2>,... that got assigned (and, somewhat related, C<$^N> to the -value of the C<$1>, C<$2>,... most-recently assigned; i.e. the C<$1>, +value of the C<$1>, C<$2>,... most-recently assigned; I the C<$1>, C<$2>,... associated with the rightmost closing parenthesis used in the match). @@ -732,21 +796,21 @@ match). =head2 Backreferences Closely associated with the matching variables C<$1>, C<$2>, ... are -the I C<\1>, C<\2>,... Backreferences are simply +the I C<\g1>, C<\g2>,... Backreferences are simply matching variables that can be used I a regexp. This is a -really nice feature -- what matches later in a regexp is made to depend on +really nice feature; what matches later in a regexp is made to depend on what matched earlier in the regexp. Suppose we wanted to look -for doubled words in a text, like 'the the'. The following regexp finds +for doubled words in a text, like "the the". The following regexp finds all 3-letter doubles with a space in between: - /\b(\w\w\w)\s\1\b/; + /\b(\w\w\w)\s\g1\b/; -The grouping assigns a value to \1, so that the same 3 letter sequence +The grouping assigns a value to C<\g1>, so that the same 3-letter sequence is used for both parts. A similar task is to find words consisting of two identical parts: - % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\1$' /usr/dict/words + % simple_grep '^(\w\w\w\w|\w\w\w|\w\w|\w)\g1$' /usr/dict/words beriberi booboo coco @@ -755,27 +819,27 @@ A similar task is to find words consisting of two identical parts: papa The regexp has a single grouping which considers 4-letter -combinations, then 3-letter combinations, etc., and uses C<\1> to look for -a repeat. Although C<$1> and C<\1> represent the same thing, care should be +combinations, then 3-letter combinations, I., and uses C<\g1> to look for +a repeat. Although C<$1> and C<\g1> represent the same thing, care should be taken to use matched variables C<$1>, C<$2>,... only I a regexp -and backreferences C<\1>, C<\2>,... only I a regexp; not doing +and backreferences C<\g1>, C<\g2>,... only I a regexp; not doing so may lead to surprising and unsatisfactory results. =head2 Relative backreferences Counting the opening parentheses to get the correct number for a -backreference is errorprone as soon as there is more than one +backreference is error-prone as soon as there is more than one capturing group. A more convenient technique became available with Perl 5.10: relative backreferences. To refer to the immediately preceding capture group one now may write C<\g{-1}>, the next but last is available via C<\g{-2}>, and so on. Another good reason in addition to readability and maintainability -for using relative backreferences is illustrated by the following example, +for using relative backreferences is illustrated by the following example, where a simple pattern for matching peculiar strings is used: - $a99a = '([a-z])(\d)\2\1'; # matches a11a, g22g, x33x, etc. + $a99a = '([a-z])(\d)\g2\g1'; # matches a11a, g22g, x33x, etc. Now that we have this pattern stored as a handy string, we might feel tempted to use it as a part of some other pattern: @@ -787,10 +851,10 @@ tempted to use it as a part of some other pattern: print "bad line: '$line'\n"; } -But this doesn't match -- at least not the way one might expect. Only +But this doesn't match, at least not the way one might expect. Only after inserting the interpolated C<$a99a> and looking at the resulting full text of the regexp is it obvious that the backreferences have -backfired -- the subexpression C<(\w+)> has snatched number 1 and +backfired. The subexpression C<(\w+)> has snatched number 1 and demoted the groups in C<$a99a> by one rank. This can be avoided by using relative backreferences: @@ -799,24 +863,24 @@ using relative backreferences: =head2 Named backreferences -Perl 5.10 also introduced named capture buffers and named backreferences. +Perl 5.10 also introduced named capture groups and named backreferences. To attach a name to a capturing group, you write either C<< (?...) >> or C<< (?'name'...) >>. The backreference may then be written as C<\g{name}>. It is permissible to attach the same name to more than one group, but then only the leftmost one of the eponymous set can be referenced. Outside of the pattern a named -capture buffer is accessible through the C<%+> hash. +capture group is accessible through the C<%+> hash. Assuming that we have to match calendar dates which may be given in one of the three formats yyyy-mm-dd, mm/dd/yyyy or dd.mm.yyyy, we can write -three suitable patterns where we use 'd', 'm' and 'y' respectively as the -names of the buffers capturing the pertaining components of a date. The +three suitable patterns where we use C<'d'>, C<'m'> and C<'y'> respectively as the +names of the groups capturing the pertaining components of a date. The matching operation combines the three patterns as alternatives: $fmt1 = '(?\d\d\d\d)-(?\d\d)-(?\d\d)'; $fmt2 = '(?\d\d)/(?\d\d)/(?\d\d\d\d)'; $fmt3 = '(?\d\d)\.(?\d\d)\.(?\d\d\d\d)'; - for my $d qw( 2006-10-21 15.01.2007 10/31/2005 ){ + for my $d (qw(2006-10-21 15.01.2007 10/31/2005)) { if ( $d =~ m{$fmt1|$fmt2|$fmt3} ){ print "day=$+{d} month=$+{m} year=$+{y}\n"; } @@ -838,22 +902,22 @@ Consider a pattern for matching a time of the day, civil or military style: Processing the results requires an additional if statement to determine whether C<$1> and C<$2> or C<$3> and C<$4> contain the goodies. It would -be easier if we could use buffer numbers 1 and 2 in second alternative as +be easier if we could use group numbers 1 and 2 in second alternative as well, and this is exactly what the parenthesized construct C<(?|...)>, set around an alternative achieves. Here is an extended version of the previous pattern: - if ( $time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/ ){ - print "hour=$1 minute=$2 zone=$3\n"; - } + if($time =~ /(?|(\d\d|\d):(\d\d)|(\d\d)(\d\d))\s+([A-Z][A-Z][A-Z])/){ + print "hour=$1 minute=$2 zone=$3\n"; + } -Within the alternative numbering group, buffer numbers start at the same +Within the alternative numbering group, group numbers start at the same position for each alternative. After the group, numbering continues with one higher than the maximum reached across all the alternatives. =head2 Position information -In addition to what was matched, Perl (since 5.6.0) also provides the +In addition to what was matched, Perl also provides the positions of what was matched as contents of the C<@-> and C<@+> arrays. C<$-[0]> is the position of the start of the entire match and C<$+[0]> is the position of the end. Similarly, C<$-[n]> is the @@ -863,8 +927,8 @@ this code $x = "Mmm...donut, thought Homer"; $x =~ /^(Mmm|Yech)\.\.\.(donut|peas)/; # matches - foreach $expr (1..$#-) { - print "Match $expr: '${$expr}' at position ($-[$expr],$+[$expr])\n"; + foreach $exp (1..$#-) { + print "Match $exp: '${$exp}' at position ($-[$exp],$+[$exp])\n"; } prints @@ -875,7 +939,7 @@ prints Even if there are no groupings in a regexp, it is still possible to find out what exactly matched in a string. If you use them, Perl will set C<$`> to the part of the string before the match, will set C<$&> -to the part of the string that matched, and will set C<$'> to the part +to the part of the string that matched, and will set C<'$'> to the part of the string after the match. An example: $x = "the cat caught the mouse"; @@ -884,7 +948,10 @@ of the string after the match. An example: In the second match, C<$`> equals C<''> 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 C<$`> and C<$'> +second "the". + +If your code is to run on Perl versions earlier than +5.20, it is worthwhile to note that using C<$`> and C<'$'> slows down regexp matching quite a bit, while C<$&> slows it down to a lesser extent, because if they are used in one regexp in a program, they are generated for I regexps in the program. So if raw @@ -896,15 +963,21 @@ C<@+> instead: $& is the same as substr( $x, $-[0], $+[0]-$-[0] ) $' is the same as substr( $x, $+[0] ) +As of Perl 5.10, the C<${^PREMATCH}>, C<${^MATCH}> and C<${^POSTMATCH}> +variables may be used. These are only set if the C

modifier is +present. Consequently they do not penalize the rest of the program. In +Perl 5.20, C<${^PREMATCH}>, C<${^MATCH}> and C<${^POSTMATCH}> are available +whether the C

has been used or not (the modifier is ignored), and +C<$`>, C<'$'> and C<$&> do not cause any speed difference. =head2 Non-capturing groupings A group that is required to bundle a set of alternatives may or may not be useful as a capturing group. If it isn't, it just creates a superfluous -addition to the set of available capture buffer values, inside as well as +addition to the set of available capture group values, inside as well as outside the regexp. Non-capturing groupings, denoted by C<(?:regexp)>, still allow the regexp to be treated as a single unit, but don't establish -a capturing buffer at the same time. Both capturing and non-capturing +a capturing group at the same time. Both capturing and non-capturing groupings are allowed to co-exist in the same regexp. Because there is no extraction, non-capturing groupings are faster than capturing groupings. Non-capturing groupings are also handy for choosing exactly @@ -924,9 +997,15 @@ elements gathered from a split operation where parentheses are required for some reason: $x = '12aba34ba5'; - @num = split /(a|b)+/, $x; # @num = ('12','a','34','b','5') + @num = split /(a|b)+/, $x; # @num = ('12','a','34','a','5') @num = split /(?:a|b)+/, $x; # @num = ('12','34','5') +In Perl 5.22 and later, all groups within a regexp can be set to +non-capturing by using the new C flag: + + "hello" =~ /(hi|hello)/n; # $1 is not set! + +See L for more information. =head2 Matching repetitions @@ -936,8 +1015,8 @@ less. We'd like to be able to match words or, more generally, strings of any length, without writing out tedious alternatives like C<\w\w\w\w|\w\w\w|\w\w|\w>. -This is exactly the problem the I metacharacters C, -C<*>, C<+>, and C<{}> were created for. They allow us to delimit the +This is exactly the problem the I metacharacters C<'?'>, +C<'*'>, C<'+'>, and C<{}> were created for. They allow us to delimit the number of repeats for 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 @@ -947,15 +1026,15 @@ meanings: =item * -C means: match 'a' 1 or 0 times +C means: match C<'a'> 1 or 0 times =item * -C means: match 'a' 0 or more times, i.e., any number of times +C means: match C<'a'> 0 or more times, I, any number of times =item * -C means: match 'a' 1 or more times, i.e., at least once +C means: match C<'a'> 1 or more times, I, at least once =item * @@ -976,15 +1055,16 @@ Here are some examples: /[a-z]+\s+\d*/; # match a lowercase word, at least one space, and # any number of digits - /(\w+)\s+\1/; # match doubled words of arbitrary length + /(\w+)\s+\g1/; # match doubled words of arbitrary length /y(es)?/i; # matches 'y', 'Y', or a case-insensitive 'yes' - $year =~ /\d{2,4}/; # make sure year is at least 2 but not more - # than 4 digits - $year =~ /\d{4}|\d{2}/; # better match; throw out 3 digit dates - $year =~ /\d{2}(\d{2})?/; # same thing written differently. However, - # this produces $1 and the other does not. - - % simple_grep '^(\w+)\1$' /usr/dict/words # isn't this easier? + $year =~ /^\d{2,4}$/; # make sure year is at least 2 but not more + # than 4 digits + $year =~ /^\d{4}$|^\d{2}$/; # better match; throw out 3-digit dates + $year =~ /^\d{2}(\d{2})?$/; # same thing written differently. + # However, this captures the last two + # digits in $1 and the other does not. + + % simple_grep '^(\w+)\g1$' /usr/dict/words # isn't this easier? beriberi booboo coco @@ -994,9 +1074,9 @@ Here are some examples: 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 C, Perl will first try to match the regexp with the C +with C, Perl will first try to match the regexp with the C<'a'> present; if that fails, Perl will try to match the regexp without the -C present. For the quantifier C<*>, we get the following: +C<'a'> present. For the quantifier C<'*'>, we get the following: $x = "the cat in the hat"; $x =~ /^(.*)(cat)(.*)$/; # matches, @@ -1017,9 +1097,9 @@ stop there, but that wouldn't give the longest possible string to the first quantifier C<.*>. Instead, the first quantifier C<.*> grabs as much of the string as possible while still having the regexp match. In this example, that means having the C sequence with the final C -in the string. The other important principle illustrated here is that +in the string. The other important principle illustrated here is that, when there are two or more elements in a regexp, the I -quantifier, if there is one, gets to grab as much the string as +quantifier, if there is one, gets to grab as much of the string as possible, leaving the rest of the regexp to fight over scraps. Thus in our example, the first quantifier C<.*> grabs most of the string, while the second quantifier C<.*> gets the empty string. Quantifiers that @@ -1043,7 +1123,7 @@ that allows a match for the whole regexp will be the one used. =item * -Principle 2: The maximal matching quantifiers C, C<*>, C<+> and +Principle 2: The maximal matching quantifiers C<'?'>, C<'*'>, C<'+'> and C<{n,m}> will in general match as much of the string as possible while still allowing the whole regexp to match. @@ -1059,7 +1139,7 @@ satisfied. =back -As we have seen above, Principle 0 overrides the others -- the regexp +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. @@ -1073,8 +1153,8 @@ Here is an example of these principles in action: # $3 = 'l' This regexp matches at the earliest string position, C<'T'>. One -might think that C, being leftmost in the alternation, would be -matched, but C produces the longest string in the first quantifier. +might think that C<'e'>, being leftmost in the alternation, would be +matched, but C<'r'> produces the longest string in the first quantifier. $x =~ /(m{1,2})(.*)$/; # matches, # $1 = 'mm' @@ -1099,7 +1179,7 @@ C<'m'> for the second quantifier C. Here, C<.?> eats its maximal one character at the earliest possible position in the string, C<'a'> in C, leaving C -the opportunity to match both C's. Finally, +the opportunity to match both C<'m'>'s. Finally, "aXXXb" =~ /(X*)/; # matches with $1 = '' @@ -1111,23 +1191,23 @@ Sometimes greed is not good. At times, we would like quantifiers to match a I piece of string, rather than a maximal piece. For this purpose, Larry Wall created the I or I quantifiers C, C<*?>, C<+?>, and C<{}?>. These are -the usual quantifiers with a C appended to them. They have the +the usual quantifiers with a C<'?'> appended to them. They have the following meanings: =over 4 =item * -C means: match 'a' 0 or 1 times. Try 0 first, then 1. +C means: match C<'a'> 0 or 1 times. Try 0 first, then 1. =item * -C means: match 'a' 0 or more times, i.e., any number of times, +C means: match C<'a'> 0 or more times, I, any number of times, but as few times as possible =item * -C means: match 'a' 1 or more times, i.e., at least once, but +C means: match C<'a'> 1 or more times, I, at least once, but as few times as possible =item * @@ -1156,9 +1236,9 @@ Let's look at the example above, but with minimal quantifiers: # $2 = 'e' # $3 = ' programming republic of Perl' -The minimal string that will allow both the start of the string C<^> +The minimal string that will allow both the start of the string C<'^'> and the alternation to match is C, with the alternation C -matching C. The second quantifier C<.*> is free to gobble up the +matching C<'e'>. The second quantifier C<.*> is free to gobble up the rest of the string. $x =~ /(m{1,2}?)(.*?)$/; # matches, @@ -1169,7 +1249,7 @@ The first string position that this regexp can match is at the first C<'m'> in C. At this position, the minimal C matches just one C<'m'>. Although the second quantifier C<.*?> would prefer to match no characters, it is constrained by the end-of-string -anchor C<$> to match the rest of the string. +anchor C<'$'> to match the rest of the string. $x =~ /(.*?)(m{1,2}?)(.*)$/; # matches, # $1 = 'The progra' @@ -1177,12 +1257,12 @@ anchor C<$> to match the rest of the string. # $3 = 'ming republic of Perl' In this regexp, you might expect the first minimal quantifier C<.*?> -to match the empty string, because it is not constrained by a C<^> +to match the empty string, because it is not constrained by a C<'^'> 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 I match at the start of the string. Thus -the first quantifier has to match everything up to the first C. The -second minimal quantifier matches just one C and the third +the first quantifier has to match everything up to the first C<'m'>. The +second minimal quantifier matches just one C<'m'> and the third quantifier matches the rest of the string. $x =~ /(.??)(m{1,2})(.*)$/; # matches, @@ -1223,37 +1303,36 @@ backtracking. Here is a step-by-step analysis of the example =over 4 -=item 0 +=item Z<>0. Start with the first letter in the string C<'t'>. -Start with the first letter in the string 't'. +E -=item 1 +=item Z<>1. The first quantifier C<'.*'> starts out by matching the whole +string "C". -The first quantifier '.*' starts out by matching the whole -string 'the cat in the hat'. +E -=item 2 +=item Z<>2. C<'a'> in the regexp element C<'at'> doesn't match the end +of the string. Backtrack one character. -'a' in the regexp element 'at' doesn't match the end of the -string. Backtrack one character. +E -=item 3 +=item Z<>3. C<'a'> in the regexp element C<'at'> still doesn't match +the last letter of the string C<'t'>, so backtrack one more character. -'a' in the regexp element 'at' still doesn't match the last -letter of the string 't', so backtrack one more character. +E -=item 4 +=item Z<>4. Now we can match the C<'a'> and the C<'t'>. -Now we can match the 'a' and the 't'. +E -=item 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. +=item Z<>5. Move on to the third element C<'.*'>. Since we are at the +end of the string and C<'.*'> can match 0 times, assign it the empty +string. -=item 6 +E -We are done! +=item Z<>6. We are done! =back @@ -1265,14 +1344,14 @@ 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 C<+> -and C<*>: one repetition with C of length n, two repetitions with +different ways of partitioning a string of length n between the C<'+'> +and C<'*'>: one repetition with C of length n, two repetitions with the first C 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 +whose bits add up to length n, I. In fact there are an exponential number of ways to partition a string as a function of its length. A regexp may get lucky and match early in the process, but if there is no match, Perl will try I possibility before giving up. So be -careful with nested C<*>'s, C<{n,m}>'s, and C<+>'s. The book +careful with nested C<'*'>'s, C<{n,m}>'s, and C<'+'>'s. The book I by Jeffrey Friedl gives a wonderful discussion of this and other efficiency issues. @@ -1287,15 +1366,15 @@ the simple pattern Whenever this is applied to a string which doesn't quite meet the pattern's expectations such as S> or S>, -the regex engine will backtrack, approximately once for each character +the regexp engine will backtrack, approximately once for each character in the string. But we know that there is no way around taking I of the initial word characters to match the first repetition, that I spaces must be eaten by the middle part, and the same goes for the second word. With the introduction of the I in Perl 5.10, we -have a way of instructing the regex engine not to backtrack, with the -usual quantifiers with a C<+> appended to them. This makes them greedy as +have a way of instructing the regexp engine not to backtrack, with the +usual quantifiers with a C<'+'> appended to them. This makes them greedy as well as stingy; once they succeed they won't give anything back to permit another solution. They have the following meanings: @@ -1383,12 +1462,12 @@ Now consider floating point numbers with exponents. The key observation here is that I 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 +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)$/; -The exponent is an C or C, followed by an integer. So the +The exponent is an C<'e'> or C<'E'>, followed by an integer. So the exponent regexp is /[eE][+-]?\d+/; # exponent @@ -1398,10 +1477,10 @@ Putting all the parts together, we get a regexp that matches numbers: /^[+-]?(\d+\.\d+|\d+\.|\.\d+|\d+)([eE][+-]?\d+)?$/; # Ta da! Long regexps like this may impress your friends, but can be hard to -decipher. In complex situations like this, the C modifier for a +decipher. In complex situations like this, the C 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 +we can rewrite our "extended" regexp in the more pleasing form /^ [+-]? # first, match an optional sign @@ -1411,13 +1490,13 @@ we can rewrite our 'extended' regexp in the more pleasing form |\.\d+ # mantissa of the form .b |\d+ # integer of the form a ) - ([eE][+-]?\d+)? # finally, optionally match an exponent + ( [eE] [+-]? \d+ )? # 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 S> or put it in a character class S>. The same thing -goes for pound signs, use C<\#> or C<[#]>. For instance, Perl allows +goes for pound signs: use C<\#> or C<[#]>. For instance, Perl allows a space between the sign and the mantissa or integer, and we could add this to our regexp as follows: @@ -1429,7 +1508,7 @@ this to our regexp as follows: |\.\d+ # mantissa of the form .b |\d+ # integer of the form a ) - ([eE][+-]?\d+)? # finally, optionally match an exponent + ( [eE] [+-]? \d+ )? # finally, optionally match an exponent $/x; In this form, it is easier to see a way to simplify the @@ -1445,10 +1524,28 @@ could be factored out: )? # ? takes care of integers of the form a |\.\d+ # mantissa of the form .b ) - ([eE][+-]?\d+)? # finally, optionally match an exponent + ( [eE] [+-]? \d+ )? # finally, optionally match an exponent $/x; -or written in the compact form, +Starting in Perl v5.26, specifying C changes the square-bracketed +portions of a pattern to ignore tabs and space characters unless they +are escaped by preceding them with a backslash. So, we could write + + /^ + [ + - ]?\ * # first, match an optional sign + ( # then match integers or f.p. mantissas: + \d+ # start out with a ... + ( + \.\d* # mantissa of the form a.b or a. + )? # ? takes care of integers of the form a + |\.\d+ # mantissa of the form .b + ) + ( [ e E ] [ + - ]? \d+ )? # finally, optionally match an exponent + $/xx; + +This doesn't really improve the legibility of this example, but it's +available in case you want it. Squashing the pattern down to the +compact form, we have /^[+-]?\ *(\d+(\.\d*)?|\.\d+)([eE][+-]?\d+)?$/; @@ -1492,35 +1589,10 @@ We have already introduced the matching operator in its default C and arbitrary delimiter C forms. We have used the binding operator C<=~> and its negation C to test for string matches. Associated with the matching operator, we have discussed the -single line C, multi-line C, case-insensitive C and -extended C modifiers. There are a few more things you might +single line C, multi-line C, case-insensitive C and +extended C modifiers. There are a few more things you might want to know about matching operators. -=head3 Optimizing pattern evaluation - -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 C. It is not as -efficient as it could be, however, because Perl has to re-evaluate -(or compile) C<$pattern> each time through the loop. If C<$pattern> won't be -changing over the lifetime of the script, we can add the C -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 - } - - =head3 Prohibiting substitution If you change C<$pattern> after the first substitution happens, Perl @@ -1533,7 +1605,7 @@ special delimiter C: } Similar to strings, C acts like apostrophes on a regexp; all other -C delimiters act like quotes. If the regexp evaluates to the empty string, +C<'m'> delimiters act like quotes. If the regexp evaluates to the empty string, the regexp in the I is used instead. So we have "dog" =~ /d/; # 'd' matches @@ -1542,15 +1614,16 @@ the regexp in the I is used instead. So we have =head3 Global matching -The final two modifiers C and C concern multiple matches. -The modifier C stands for global matching and allows the +The final two modifiers we will discuss here, +C and C, concern multiple matches. +The modifier C 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 -`C jump from match to match, keeping track of position in the +C 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 C function. -The use of C is shown in the following example. Suppose we have +The use of C 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: @@ -1562,7 +1635,7 @@ groupings: # $3 = 'house' But what if we had an indeterminate number of words? This is the sort -of task C was made for. To extract all words, form the simple +of task C was made for. To extract all words, form the simple regexp C<(\w+)> and loop over all matches with C: while ($x =~ /(\w+)/g) { @@ -1577,22 +1650,22 @@ prints A failed match or changing the target string resets the position. If you don't want the position reset after failure to match, add the -C, as in C. The current position in the string is +C, as in C. The current position in the string is associated with the string, not the regexp. This means that different strings have different positions and their respective positions can be set or read independently. -In list context, C returns a list of matched groupings, or if +In list context, C returns a list of matched groupings, or if there are no groupings, a list of matches to the whole regexp. So if we wanted just the words, we could use @words = ($x =~ /(\w+)/g); # matches, - # $word[0] = 'cat' - # $word[1] = 'dog' - # $word[2] = 'house' + # $words[0] = 'cat' + # $words[1] = 'dog' + # $words[2] = 'house' -Closely associated with the C modifier is the C<\G> anchor. The -C<\G> anchor matches at the point where the previous C match left +Closely associated with the C modifier is the C<\G> anchor. The +C<\G> anchor matches at the point where the previous C match left off. C<\G> allows us to easily do context-sensitive matching: $metric = 1; # use metric units @@ -1608,12 +1681,12 @@ off. C<\G> allows us to easily do context-sensitive matching: } $x =~ /\G\s+(widget|sprocket)/g; # continue processing -The combination of C and C<\G> allows us to process the string a +The combination of C and C<\G> allows us to process the string a bit at a time and use arbitrary Perl logic to decide what to do next. Currently, the C<\G> anchor is only fully supported when used to anchor to the start of the pattern. -C<\G> is also invaluable in processing fixed length records with +C<\G> is also invaluable in processing fixed-length records with regexps. Suppose we have a snippet of coding region DNA, encoded as base pair letters C and we want to find all the stop codons C. In a coding region, codons are 3-letter sequences, so @@ -1625,7 +1698,7 @@ naive regexp $dna =~ /TGA/; doesn't work; it may match a C, but there is no guarantee that -the match is aligned with codon boundaries, e.g., the substring +the match is aligned with codon boundaries, I, the substring S> gives a match. A better solution is while ($dna =~ /(\w\w\w)*?TGA/g) { # note the minimal *? @@ -1657,6 +1730,10 @@ which is the correct answer. This example illustrates that it is important not only to match what is desired, but to reject what is not desired. +(There are other regexp modifiers that are available, such as +C, but their specialized uses are beyond the +scope of this introduction. ) + =head3 Search and replace Regular expressions also play a big role in I @@ -1664,11 +1741,11 @@ operations in Perl. Search and replace is accomplished with the C operator. The general form is C, with everything we know about regexps and modifiers applying in this case as well. The -C is a Perl double quoted string that replaces in the +I is a Perl double-quoted string that replaces in the string whatever is matched with the C. The operator C<=~> is also used here to associate a string with C. If matching against C<$_>, the S> can be dropped. If there is a match, -C returns the number of substitutions made, otherwise it returns +C returns the number of substitutions made; otherwise it returns false. Here are a few examples: $x = "Time to feed the cat!"; @@ -1682,7 +1759,7 @@ false. Here are a few examples: In the last example, the whole string was matched, but only the part inside the single quotes was grouped. With the C operator, the -matched variables C<$1>, C<$2>, etc. are immediately available for use +matched variables C<$1>, C<$2>, I. are immediately available for use in the replacement expression, so we use C<$1> to replace the quoted string with just what was quoted. With the global modifier, C will search and replace all occurrences of the regexp in the string: @@ -1694,7 +1771,7 @@ will search and replace all occurrences of the regexp in the string: $x =~ s/4/four/g; # does it all: # $x contains "I batted four for four" -If you prefer 'regex' over 'regexp' in this tutorial, you could use +If you prefer "regex" over "regexp" in this tutorial, you could use the following program to replace it: % cat > simple_replace @@ -1702,7 +1779,7 @@ the following program to replace it: $regexp = shift; $replacement = shift; while (<>) { - s/$regexp/$replacement/go; + s/$regexp/$replacement/g; print; } ^D @@ -1710,19 +1787,48 @@ the following program to replace it: % simple_replace regexp regex perlretut.pod In C we used the C modifier to replace all -occurrences of the regexp on each line and the C modifier to -compile the regexp only once. As with C, both the -C and the C use C<$_> implicitly. +occurrences of the regexp on each line. (Even though the regular +expression appears in a loop, Perl is smart enough to compile it +only once.) As with C, both the +C and the C use C<$_> implicitly. + +If you don't want C to change your original variable you can use +the non-destructive substitute modifier, C. This changes the +behavior so that C returns the final substituted string +(instead of the number of substitutions): + + $x = "I like dogs."; + $y = $x =~ s/dogs/cats/r; + print "$x $y\n"; + +That example will print "I like dogs. I like cats". Notice the original +C<$x> variable has not been affected. The overall +result of the substitution is instead stored in C<$y>. If the +substitution doesn't affect anything then the original string is +returned: + + $x = "I like dogs."; + $y = $x =~ s/elephants/cougars/r; + print "$x $y\n"; # prints "I like dogs. I like dogs." + +One other interesting thing that the C flag allows is chaining +substitutions: + + $x = "Cats are great."; + print $x =~ s/Cats/Dogs/r =~ s/Dogs/Frogs/r =~ + s/Frogs/Hedgehogs/r, "\n"; + # prints "Hedgehogs are great." A modifier available specifically to search and replace is the -C evaluation modifier. C wraps an C around -the replacement string and the evaluated result is substituted for the +C evaluation modifier. C treats the +replacement text as Perl code, rather than a double-quoted +string. The value that the code returns is substituted for the matched substring. C is useful if you need to do a bit of computation in the process of replacing text. This example counts character frequencies in a line: $x = "Bill the cat"; - $x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself + $x =~ s/(.)/$chars{$1}++;$1/eg; # final $1 replaces char with itself print "frequency of '$_' is $chars{$_}\n" foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars); @@ -1740,9 +1846,10 @@ This prints As with the match C operator, C can use other delimiters, such as C and C, and even C. If single quotes are -used C, then the regexp and replacement are treated as single -quoted strings and there are no substitutions. C in list context -returns the same thing as in scalar context, i.e., the number of +used C, then the regexp and replacement are +treated as single-quoted strings and there are no +variable substitutions. C in list context +returns the same thing as in scalar context, I, the number of matches. =head3 The split function @@ -1777,13 +1884,13 @@ groupings as well. For instance, # $parts[5] = '/' # $parts[6] = 'perl' -Since the first character of $x matched the regexp, C prepended +Since the first character of C<$x> matched the regexp, C prepended an empty initial element to the list. If you have read this far, congratulations! You now have all the basic tools needed to use regular expressions to solve a wide range of text processing problems. If this is your first time through the tutorial, -why not stop here and play around with regexps a while... S +why not stop here and play around with regexps a while.... S concerns the more esoteric aspects of regular expressions and those concepts certainly aren't needed right at the start. @@ -1798,7 +1905,7 @@ too often on a hike, but when we are stuck, they can be invaluable. What follows are the more advanced, less used, or sometimes esoteric capabilities of Perl regexps. In Part 2, we will assume you are -comfortable with the basics and concentrate on the new features. +comfortable with the basics and concentrate on the advanced features. =head2 More on characters, strings, and character classes @@ -1836,24 +1943,31 @@ instance, $x = "\QThat !^*&%~& cat!"; $x =~ /\Q!^*&%~&\E/; # check for rough language -It does not protect C<$> or C<@>, so that variables can still be +It does not protect C<'$'> or C<'@'>, so that variables can still be substituted. -With the advent of 5.6.0, Perl regexps can handle more than just the -standard ASCII character set. Perl now supports I, a standard +C<\Q>, C<\L>, C<\l>, C<\U>, C<\u> and C<\E> are actually part of +double-quotish syntax, and not part of regexp syntax proper. They will +work if they appear in a regular expression embedded directly in a +program, but not when contained in a string that is interpolated in a +pattern. + +Perl regexps can handle more than just the +standard ASCII character set. Perl supports I, a standard for representing the alphabets from virtually all of the world's written languages, and a host of symbols. Perl's text strings are Unicode strings, so they can contain characters with a value (codepoint or character number) higher -than 255 +than 255. What does this mean for regexps? Well, regexp users don't need to know much about Perl's internal representation of strings. But they do need to know 1) how to represent Unicode characters in a regexp and 2) that a matching operation will treat the string to be searched as a sequence of characters, not bytes. The answer to 1) is that Unicode characters -greater than C are represented using the C<\x{hex}> notation, -because the \0 octal and \x hex (without curly braces) don't go further -than 255. +greater than C are represented using the C<\x{hex}> notation, because +C<\x>I (without curly braces and I are two hex digits) doesn't +go further than 255. (Starting in Perl 5.14, if you're an octal fan, +you can also use C<\o{oct}>.) /\x{263a}/; # match a Unicode smiley face :) @@ -1866,121 +1980,119 @@ Unicode and encoded in UTF-8, then an explicit C is needed.) Figuring out the hexadecimal sequence of a Unicode character you want or deciphering someone else's hexadecimal Unicode regexp is about as much fun as programming in machine code. So another way to specify -Unicode characters is to use the I> escape -sequence C<\N{name}>. C is a name for the Unicode character, as +Unicode characters is to use the I escape +sequence C<\N{I}>. I is a name for the Unicode character, as specified in the Unicode standard. For instance, if we wanted to represent or match the astrological sign for the planet Mercury, we could use - use charnames ":full"; # use named chars with Unicode full names $x = "abc\N{MERCURY}def"; $x =~ /\N{MERCURY}/; # matches -One can also use short names or restrict names to a certain alphabet: +One can also use "short" names: - use charnames ':full'; print "\N{GREEK SMALL LETTER SIGMA} is called sigma.\n"; - - use charnames ":short"; print "\N{greek:Sigma} is an upper-case sigma.\n"; +You can also restrict names to a certain alphabet by specifying the +L pragma: + use charnames qw(greek); print "\N{sigma} is Greek sigma\n"; -A list of full names is found in the file NamesList.txt in the -lib/perl5/X.X.X/unicore directory (where X.X.X is the perl -version number as it is installed on your system). +An index of character names is available on-line from the Unicode +Consortium, L; explanatory +material with links to other resources at +L. + +The answer to requirement 2) is that a regexp (mostly) +uses Unicode characters. The "mostly" is for messy backward +compatibility reasons, but starting in Perl 5.14, any regexp compiled in +the scope of a C (which is automatically +turned on within the scope of a C or higher) will turn that +"mostly" into "always". If you want to handle Unicode properly, you +should ensure that C<'unicode_strings'> is turned on. +Internally, this is encoded to bytes using either UTF-8 or a native 8 +bit encoding, depending on the history of the string, but conceptually +it is a sequence of characters, not bytes. See L for a +tutorial about that. + +Let us now discuss Unicode character classes, most usually called +"character properties". These are represented by the C<\p{I}> +escape sequence. The negation of this is C<\P{I}>. For example, +to match lower and uppercase characters, -The answer to requirement 2), as of 5.6.0, is that a regexp uses Unicode -characters. Internally, this is encoded to bytes using either UTF-8 or a -native 8 bit encoding, depending on the history of the string, but -conceptually it is a sequence of characters, not bytes. See -L for a tutorial about that. - -Let us now discuss Unicode character classes. Just as with Unicode -characters, there are named Unicode character classes represented by the -C<\p{name}> escape sequence. Closely associated is the C<\P{name}> -character class, which is the negation of the C<\p{name}> class. For -example, to match lower and uppercase characters, - - use charnames ":full"; # use named chars with Unicode full names $x = "BOB"; $x =~ /^\p{IsUpper}/; # matches, uppercase char class $x =~ /^\P{IsUpper}/; # doesn't match, char class sans uppercase $x =~ /^\p{IsLower}/; # doesn't match, lowercase char class $x =~ /^\P{IsLower}/; # matches, char class sans lowercase -Here is the association between some Perl named classes and the -traditional Unicode classes: - - Perl class name Unicode class name or regular expression - - IsAlpha /^[LM]/ - IsAlnum /^[LMN]/ - IsASCII $code <= 127 - IsCntrl /^C/ - IsBlank $code =~ /^(0020|0009)$/ || /^Z[^lp]/ - IsDigit Nd - IsGraph /^([LMNPS]|Co)/ - IsLower Ll - IsPrint /^([LMNPS]|Co|Zs)/ - IsPunct /^P/ - IsSpace /^Z/ || ($code =~ /^(0009|000A|000B|000C|000D)$/ - IsSpacePerl /^Z/ || ($code =~ /^(0009|000A|000C|000D|0085|2028|2029)$/ - IsUpper /^L[ut]/ - IsWord /^[LMN]/ || $code eq "005F" - IsXDigit $code =~ /^00(3[0-9]|[46][1-6])$/ - -You can also use the official Unicode class names with the C<\p> and -C<\P>, like C<\p{L}> for Unicode 'letters', or C<\p{Lu}> for uppercase -letters, or C<\P{Nd}> for non-digits. If a C is just one -letter, the braces can be dropped. For instance, C<\pM> is the -character class of Unicode 'marks', for example accent marks. -For the full list see L. - -The Unicode has also been separated into various sets of characters -which you can test with C<\p{...}> (in) and C<\P{...}> (not in). -To test whether a character is (or is not) an element of a script -you would use the script name, for example C<\p{Latin}>, C<\p{Greek}>, -or C<\P{Katakana}>. Other sets are the Unicode blocks, the names -of which begin with "In". One such block is dedicated to mathematical -operators, and its pattern formula is }>. -For the full list see L. +(The "C" is optional.) + +There are many, many Unicode character properties. For the full list +see L. Most of them have synonyms with shorter names, +also listed there. Some synonyms are a single character. For these, +you can drop the braces. For instance, C<\pM> is the same thing as +C<\p{Mark}>, meaning things like accent marks. + +The Unicode C<\p{Script}> and C<\p{Script_Extensions}> properties are +used to categorize every Unicode character into the language script it +is written in. (C is an improved version of +C