Text::Balanced(3p) Perl Programmers Reference Guide Text::Balanced(3p) #
Text::Balanced(3p) Perl Programmers Reference Guide Text::Balanced(3p)
NNAAMMEE #
Text::Balanced - Extract delimited text sequences from strings.
SSYYNNOOPPSSIISS #
use Text::Balanced qw (
extract_delimited
extract_bracketed
extract_quotelike
extract_codeblock
extract_variable
extract_tagged
extract_multiple
gen_delimited_pat
gen_extract_tagged
);
# Extract the initial substring of $text that is delimited by
# two (unescaped) instances of the first character in $delim.
($extracted, $remainder) = extract_delimited($text,$delim);
# Extract the initial substring of $text that is bracketed
# with a delimiter(s) specified by $delim (where the string
# in $delim contains one or more of '(){}[]<>').
($extracted, $remainder) = extract_bracketed($text,$delim);
# Extract the initial substring of $text that is bounded by
# an XML tag.
($extracted, $remainder) = extract_tagged($text);
# Extract the initial substring of $text that is bounded by
# a C<BEGIN>...C<END> pair. Don't allow nested C<BEGIN> tags
($extracted, $remainder) =
extract_tagged($text,"BEGIN","END",undef,{bad=>["BEGIN"]});
# Extract the initial substring of $text that represents a
# Perl "quote or quote-like operation"
($extracted, $remainder) = extract_quotelike($text);
# Extract the initial substring of $text that represents a block
# of Perl code, bracketed by any of character(s) specified by $delim
# (where the string $delim contains one or more of '(){}[]<>').
($extracted, $remainder) = extract_codeblock($text,$delim);
# Extract the initial substrings of $text that would be extracted by
# one or more sequential applications of the specified functions
# or regular expressions
@extracted = extract_multiple($text,
[ \&extract_bracketed,
\&extract_quotelike,
\&some_other_extractor_sub,
qr/[xyz]*/,
'literal',
]);
# Create a string representing an optimized pattern (a la Friedl)
# that matches a substring delimited by any of the specified characters
# (in this case: any type of quote or a slash)
$patstring = gen_delimited_pat(q{'"`/});
# Generate a reference to an anonymous sub that is just like extract_tagged
# but pre-compiled and optimized for a specific pair of tags, and
# consequently much faster (i.e. 3 times faster). It uses qr// for better
# performance on repeated calls.
$extract_head = gen_extract_tagged('<HEAD>','</HEAD>');
($extracted, $remainder) = $extract_head->($text);
DDEESSCCRRIIPPTTIIOONN #
The various "extract_..." subroutines may be used to extract a delimited
substring, possibly after skipping a specified prefix string. By default,
that prefix is optional whitespace ("/\s*/"), but you can change it to
whatever you wish (see below).
The substring to be extracted must appear at the current "pos" location
of the string's variable (or at index zero, if no "pos" position is
defined). In other words, the "extract_..." subroutines _d_o_n_'_t extract
the first occurrence of a substring anywhere in a string (like an
unanchored regex would). Rather, they extract an occurrence of the
substring appearing immediately at the current matching position in the
string (like a "\G"-anchored regex would).
GGeenneerraall BBeehhaavviioouurr iinn LLiisstt CCoonntteexxttss In a list context, all the subroutines return a list, the first three elements of which are always:
[0] The extracted string, including the specified delimiters. If the
extraction fails "undef" is returned.
[1] The remainder of the input string (i.e. the characters after the
extracted string). On failure, the entire string is returned.
[2] The skipped prefix (i.e. the characters before the extracted string).
On failure, "undef" is returned.
Note that in a list context, the contents of the original input text (the
first argument) are not modified in any way.
However, if the input text was passed in a variable, that variable's
"pos" value is updated to point at the first character after the
extracted text. That means that in a list context the various subroutines
can be used much like regular expressions. For example:
while ( $next = (extract_quotelike($text))[0] )
{
# process next quote-like (in $next)
}
GGeenneerraall BBeehhaavviioouurr iinn SSccaallaarr aanndd VVooiidd CCoonntteexxttss In a scalar context, the extracted string is returned, having first been removed from the input text. Thus, the following code also processes each quote-like operation, but actually removes them from $text:
while ( $next = extract_quotelike($text) )
{
# process next quote-like (in $next)
}
Note that if the input text is a read-only string (i.e. a literal), no
attempt is made to remove the extracted text.
In a void context the behaviour of the extraction subroutines is exactly
the same as in a scalar context, except (of course) that the extracted
substring is not returned.
AA NNoottee AAbboouutt PPrreeffiixxeess Prefix patterns are matched without any trailing modifiers ("/gimsox" etc.) This can bite you if you’re expecting a prefix specification like ‘.*?(?=
)’ to skip everything up to the firsttag. Such a prefix
pattern will only succeed if thetag is on the current line, since .
normally doesn’t match newlines. To overcome this limitation, you need to turn on /s matching within the
prefix pattern, using the "(?s)" directive: '(?s).*?(?=<H1>)'
tag is on the current line, since .
normally doesn’t match newlines. To overcome this limitation, you need to turn on /s matching within the
prefix pattern, using the "(?s)" directive: '(?s).*?(?=<H1>)'
To overcome this limitation, you need to turn on /s matching within the
prefix pattern, using the "(?s)" directive: '(?s).*?(?=<H1>)'
FFuunnccttiioonnss “extract_delimited” The “extract_delimited” function formalizes the common idiom of extracting a single-character-delimited substring from the start of a string. For example, to extract a single-quote delimited string, the following code is typically used:
($remainder = $text) =~ s/\A('(\\.|[^'])*')//s;
$extracted = $1;
but with "extract_delimited" it can be simplified to:
($extracted,$remainder) = extract_delimited($text, "'");
"extract_delimited" takes up to four scalars (the input text, the
delimiters, a prefix pattern to be skipped, and any escape
characters) and extracts the initial substring of the text that is
appropriately delimited. If the delimiter string has multiple
characters, the first one encountered in the text is taken to delimit
the substring. The third argument specifies a prefix pattern that is
to be skipped (but must be present!) before the substring is
extracted. The final argument specifies the escape character to be
used for each delimiter.
All arguments are optional. If the escape characters are not
specified, every delimiter is escaped with a backslash ("\"). If the
prefix is not specified, the pattern '\s*' - optional whitespace - is
used. If the delimiter set is also not specified, the set "/["'`]/"
is used. If the text to be processed is not specified either, $_ is
used.
In list context, "extract_delimited" returns a array of three
elements, the extracted substring (_i_n_c_l_u_d_i_n_g _t_h_e _s_u_r_r_o_u_n_d_i_n_g
_d_e_l_i_m_i_t_e_r_s), the remainder of the text, and the skipped prefix (if
any). If a suitable delimited substring is not found, the first
element of the array is the empty string, the second is the complete
original text, and the prefix returned in the third element is an
empty string.
In a scalar context, just the extracted substring is returned. In a
void context, the extracted substring (and any prefix) are simply
removed from the beginning of the first argument.
Examples:
# Remove a single-quoted substring from the very beginning of $text:
$substring = extract_delimited($text, "'", '');
# Remove a single-quoted Pascalish substring (i.e. one in which
# doubling the quote character escapes it) from the very
# beginning of $text:
$substring = extract_delimited($text, "'", '', "'");
# Extract a single- or double- quoted substring from the
# beginning of $text, optionally after some whitespace
# (note the list context to protect $text from modification):
($substring) = extract_delimited $text, q{"'};
# Delete the substring delimited by the first '/' in $text:
$text = join '', (extract_delimited($text,'/','[^/]*')[2,1];
Note that this last example is _n_o_t the same as deleting the first
quote-like pattern. For instance, if $text contained the string:
"if ('./cmd' =~ m/$UNIXCMD/s) { $cmd = $1; }"
then after the deletion it would contain:
"if ('.$UNIXCMD/s) { $cmd = $1; }"
not:
"if ('./cmd' =~ ms) { $cmd = $1; }"
See "extract_quotelike" for a (partial) solution to this problem.
"extract_bracketed"
Like "extract_delimited", the "extract_bracketed" function takes up
to three optional scalar arguments: a string to extract from, a
delimiter specifier, and a prefix pattern. As before, a missing
prefix defaults to optional whitespace and a missing text defaults to
$_. However, a missing delimiter specifier defaults to '{}()[]<>'
(see below).
"extract_bracketed" extracts a balanced-bracket-delimited substring
(using any one (or more) of the user-specified delimiter brackets:
'(..)', '{..}', '[..]', or '<..>'). Optionally it will also respect
quoted unbalanced brackets (see below).
A "delimiter bracket" is a bracket in list of delimiters passed as
"extract_bracketed"'s second argument. Delimiter brackets are
specified by giving either the left or right (or both!) versions of
the required bracket(s). Note that the order in which two or more
delimiter brackets are specified is not significant.
A "balanced-bracket-delimited substring" is a substring bounded by
matched brackets, such that any other (left or right) delimiter
bracket _w_i_t_h_i_n the substring is also matched by an opposite (right or
left) delimiter bracket _a_t _t_h_e _s_a_m_e _l_e_v_e_l _o_f _n_e_s_t_i_n_g. Any type of
bracket not in the delimiter list is treated as an ordinary
character.
In other words, each type of bracket specified as a delimiter must be
balanced and correctly nested within the substring, and any other
kind of ("non-delimiter") bracket in the substring is ignored.
For example, given the string:
$text = "{ an '[irregularly :-(] {} parenthesized >:-)' string }";
then a call to "extract_bracketed" in a list context:
@result = extract_bracketed( $text, '{}' );
would return:
( "{ an '[irregularly :-(] {} parenthesized >:-)' string }" , "" , "" )
since both sets of '{..}' brackets are properly nested and evenly
balanced. (In a scalar context just the first element of the array
would be returned. In a void context, $text would be replaced by an
empty string.)
Likewise the call in:
@result = extract_bracketed( $text, '{[' );
would return the same result, since all sets of both types of
specified delimiter brackets are correctly nested and balanced.
However, the call in:
@result = extract_bracketed( $text, '{([<' );
would fail, returning:
( undef , "{ an '[irregularly :-(] {} parenthesized >:-)' string }" );
because the embedded pairs of '(..)'s and '[..]'s are "cross-nested"
and the embedded '>' is unbalanced. (In a scalar context, this call
would return an empty string. In a void context, $text would be
unchanged.)
Note that the embedded single-quotes in the string don't help in this
case, since they have not been specified as acceptable delimiters and
are therefore treated as non-delimiter characters (and ignored).
However, if a particular species of quote character is included in
the delimiter specification, then that type of quote will be
correctly handled. for example, if $text is:
$text = '<A HREF=">>>>">link</A>';
then
@result = extract_bracketed( $text, '<">' );
returns:
( '<A HREF=">>>>">', 'link</A>', "" )
as expected. Without the specification of """ as an embedded quoter:
@result = extract_bracketed( $text, '<>' );
the result would be:
( '<A HREF=">', '>>>">link</A>', "" )
In addition to the quote delimiters "'", """, and "`", full Perl
quote-like quoting (i.e. q{string}, qq{string}, etc) can be specified
by including the letter 'q' as a delimiter. Hence:
@result = extract_bracketed( $text, '<q>' );
would correctly match something like this:
$text = '<leftop: conj /and/ conj>';
See also: "extract_quotelike" and "extract_codeblock".
"extract_variable"
"extract_variable" extracts any valid Perl variable or variable-
involved expression, including scalars, arrays, hashes, array
accesses, hash look-ups, method calls through objects, subroutine
calls through subroutine references, etc.
The subroutine takes up to two optional arguments:
1. A string to be processed ($_ if the string is omitted or "undef")
2. A string specifying a pattern to be matched as a prefix (which is
to be skipped). If omitted, optional whitespace is skipped.
On success in a list context, an array of 3 elements is returned. The
elements are:
[0] the extracted variable, or variablish expression
[1] the remainder of the input text,
[2] the prefix substring (if any),
On failure, all of these values (except the remaining text) are
"undef".
In a scalar context, "extract_variable" returns just the complete
substring that matched a variablish expression. "undef" is returned
on failure. In addition, the original input text has the returned
substring (and any prefix) removed from it.
In a void context, the input text just has the matched substring (and
any specified prefix) removed.
"extract_tagged"
"extract_tagged" extracts and segments text between (balanced)
specified tags.
The subroutine takes up to five optional arguments:
1. A string to be processed ($_ if the string is omitted or "undef")
2. A string specifying a pattern to be matched as the opening tag.
If the pattern string is omitted (or "undef") then a pattern that
matches any standard XML tag is used.
3. A string specifying a pattern to be matched at the closing tag.
If the pattern string is omitted (or "undef") then the closing
tag is constructed by inserting a "/" after any leading bracket
characters in the actual opening tag that was matched (_n_o_t the
pattern that matched the tag). For example, if the opening tag
pattern is specified as '{{\w+}}' and actually matched the
opening tag "{{DATA}}", then the constructed closing tag would be
“{{/DATA}}”. #
4. A string specifying a pattern to be matched as a prefix (which is
to be skipped). If omitted, optional whitespace is skipped.
5. A hash reference containing various parsing options (see below)
The various options that can be specified are:
"reject => $listref"
The list reference contains one or more strings specifying
patterns that must _n_o_t appear within the tagged text.
For example, to extract an HTML link (which should not contain
nested links) use:
extract_tagged($text, '<A>', '</A>', undef, {reject => ['<A>']} );
"ignore => $listref"
The list reference contains one or more strings specifying
patterns that are _n_o_t to be treated as nested tags within the
tagged text (even if they would match the start tag pattern).
For example, to extract an arbitrary XML tag, but ignore "empty"
elements:
extract_tagged($text, undef, undef, undef, {ignore => ['<[^>]*/>']} );
(also see "gen_delimited_pat" below).
"fail => $str"
The "fail" option indicates the action to be taken if a matching
end tag is not encountered (i.e. before the end of the string or
some "reject" pattern matches). By default, a failure to match a
closing tag causes "extract_tagged" to immediately fail.
However, if the string value associated with <reject> is "MAX",
then "extract_tagged" returns the complete text up to the point
of failure. If the string is "PARA", "extract_tagged" returns
only the first paragraph after the tag (up to the first line that
is either empty or contains only whitespace characters). If the
string is "", the default behaviour (i.e. failure) is reinstated.
For example, suppose the start tag "/para" introduces a
paragraph, which then continues until the next "/endpara" tag or
until another "/para" tag is encountered:
$text = "/para line 1\n\nline 3\n/para line 4";
extract_tagged($text, '/para', '/endpara', undef,
{reject => '/para', fail => MAX );
# EXTRACTED: "/para line 1\n\nline 3\n"
Suppose instead, that if no matching "/endpara" tag is found, the
"/para" tag refers only to the immediately following paragraph:
$text = "/para line 1\n\nline 3\n/para line 4";
extract_tagged($text, '/para', '/endpara', undef,
{reject => '/para', fail => MAX );
# EXTRACTED: "/para line 1\n"
Note that the specified "fail" behaviour applies to nested tags
as well.
On success in a list context, an array of 6 elements is returned. The
elements are:
[0] the extracted tagged substring (including the outermost tags),
[1] the remainder of the input text,
[2] the prefix substring (if any),
[3] the opening tag
[4] the text between the opening and closing tags
[5] the closing tag (or "" if no closing tag was found)
On failure, all of these values (except the remaining text) are
"undef".
In a scalar context, "extract_tagged" returns just the complete
substring that matched a tagged text (including the start and end
tags). "undef" is returned on failure. In addition, the original
input text has the returned substring (and any prefix) removed from
it.
In a void context, the input text just has the matched substring (and
any specified prefix) removed.
"gen_extract_tagged"
"gen_extract_tagged" generates a new anonymous subroutine which
extracts text between (balanced) specified tags. In other words, it
generates a function identical in function to "extract_tagged".
The difference between "extract_tagged" and the anonymous subroutines
generated by "gen_extract_tagged", is that those generated
subroutines:
• do not have to reparse tag specification or parsing options every
time they are called (whereas "extract_tagged" has to effectively
rebuild its tag parser on every call);
• make use of the new qr// construct to pre-compile the regexes
they use (whereas "extract_tagged" uses standard string variable
interpolation to create tag-matching patterns).
The subroutine takes up to four optional arguments (the same set as
"extract_tagged" except for the string to be processed). It returns a
reference to a subroutine which in turn takes a single argument (the
text to be extracted from).
In other words, the implementation of "extract_tagged" is exactly
equivalent to:
sub extract_tagged
{
my $text = shift;
$extractor = gen_extract_tagged(@_);
return $extractor->($text);
}
(although "extract_tagged" is not currently implemented that way).
Using "gen_extract_tagged" to create extraction functions for
specific tags is a good idea if those functions are going to be
called more than once, since their performance is typically twice as
good as the more general-purpose "extract_tagged".
"extract_quotelike"
"extract_quotelike" attempts to recognize, extract, and segment any
one of the various Perl quotes and quotelike operators (see
ppeerrlloopp(3)) Nested backslashed delimiters, embedded balanced bracket
delimiters (for the quotelike operators), and trailing modifiers are
all caught. For example, in:
extract_quotelike 'q # an octothorpe: \# (not the end of the q!) #'
extract_quotelike ' "You said, \"Use sed\"." '
extract_quotelike ' s{([A-Z]{1,8}\.[A-Z]{3})} /\L$1\E/; '
extract_quotelike ' tr/\\\/\\\\/\\\//ds; '
the full Perl quotelike operations are all extracted correctly.
Note too that, when using the /x modifier on a regex, any comment
containing the current pattern delimiter will cause the regex to be
immediately terminated. In other words:
'm /
(?i) # CASE INSENSITIVE
[a-z_] # LEADING ALPHABETIC/UNDERSCORE
[a-z0-9]* # FOLLOWED BY ANY NUMBER OF ALPHANUMERICS
/x'
will be extracted as if it were:
'm /
(?i) # CASE INSENSITIVE
[a-z_] # LEADING ALPHABETIC/'
This behaviour is identical to that of the actual compiler.
"extract_quotelike" takes two arguments: the text to be processed and
a prefix to be matched at the very beginning of the text. If no
prefix is specified, optional whitespace is the default. If no text
is given, $_ is used.
In a list context, an array of 11 elements is returned. The elements
are:
[0] the extracted quotelike substring (including trailing modifiers),
[1] the remainder of the input text,
[2] the prefix substring (if any),
[3] the name of the quotelike operator (if any),
[4] the left delimiter of the first block of the operation,
[5] the text of the first block of the operation (that is, the
contents of a quote, the regex of a match or substitution or the
target list of a translation),
[6] the right delimiter of the first block of the operation,
[7] the left delimiter of the second block of the operation (that is,
if it is a "s", "tr", or "y"),
[8] the text of the second block of the operation (that is, the
replacement of a substitution or the translation list of a
translation),
[9] the right delimiter of the second block of the operation (if
any),
[10]
the trailing modifiers on the operation (if any).
For each of the fields marked "(if any)" the default value on success
is an empty string. On failure, all of these values (except the
remaining text) are "undef".
In a scalar context, "extract_quotelike" returns just the complete
substring that matched a quotelike operation (or "undef" on failure).
In a scalar or void context, the input text has the same substring
(and any specified prefix) removed.
Examples:
# Remove the first quotelike literal that appears in text
$quotelike = extract_quotelike($text,'.*?');
# Replace one or more leading whitespace-separated quotelike
# literals in $_ with "<QLL>"
do { $_ = join '<QLL>', (extract_quotelike)[2,1] } until $@;
# Isolate the search pattern in a quotelike operation from $text
($op,$pat) = (extract_quotelike $text)[3,5];
if ($op =~ /[ms]/)
{
print "search pattern: $pat\n";
}
else
{
print "$op is not a pattern matching operation\n";
}
"extract_quotelike"
"extract_quotelike" can successfully extract "here documents" from an
input string, but with an important caveat in list contexts.
Unlike other types of quote-like literals, a here document is rarely
a contiguous substring. For example, a typical piece of code using
here document might look like this:
<<'EOMSG' || die;
This is the message.
EOMSG #
exit;
Given this as an input string in a scalar context,
"extract_quotelike" would correctly return the string
"<<'EOMSG'\nThis is the message.\nEOMSG", leaving the string " ||
die;\nexit;" in the original variable. In other words, the two
separate pieces of the here document are successfully extracted and
concatenated.
In a list context, "extract_quotelike" would return the list
[0] "<<'EOMSG'\nThis is the message.\nEOMSG\n" (i.e. the full
extracted here document, including fore and aft delimiters),
[1] " || die;\nexit;" (i.e. the remainder of the input text,
concatenated),
[2] "" (i.e. the prefix substring -- trivial in this case),
[3] "<<" (i.e. the "name" of the quotelike operator)
[4] "'EOMSG'" (i.e. the left delimiter of the here document,
including any quotes),
[5] "This is the message.\n" (i.e. the text of the here document),
[6] "EOMSG" (i.e. the right delimiter of the here document),
[7..10]
"" (a here document has no second left delimiter, second text,
second right delimiter, or trailing modifiers).
However, the matching position of the input variable would be set to
"exit;" (i.e. _a_f_t_e_r the closing delimiter of the here document),
which would cause the earlier " || die;\nexit;" to be skipped in any
sequence of code fragment extractions.
To avoid this problem, when it encounters a here document whilst
extracting from a modifiable string, "extract_quotelike" silently
rearranges the string to an equivalent piece of Perl:
«‘EOMSG’ #
This is the message.
EOMSG #
|| die;
exit;
in which the here document _i_s contiguous. It still leaves the
matching position after the here document, but now the rest of the
line on which the here document starts is not skipped.
To prevent <extract_quotelike> from mucking about with the input in
this way (this is the only case where a list-context
"extract_quotelike" does so), you can pass the input variable as an
interpolated literal:
$quotelike = extract_quotelike("$var");
"extract_codeblock"
"extract_codeblock" attempts to recognize and extract a balanced
bracket delimited substring that may contain unbalanced brackets
inside Perl quotes or quotelike operations. That is,
"extract_codeblock" is like a combination of "extract_bracketed" and
"extract_quotelike".
"extract_codeblock" takes the same initial three parameters as
"extract_bracketed": a text to process, a set of delimiter brackets
to look for, and a prefix to match first. It also takes an optional
fourth parameter, which allows the outermost delimiter brackets to be
specified separately (see below).
Omitting the first argument (input text) means process $_ instead.
Omitting the second argument (delimiter brackets) indicates that only
'{' is to be used. Omitting the third argument (prefix argument)
implies optional whitespace at the start. Omitting the fourth
argument (outermost delimiter brackets) indicates that the value of
the second argument is to be used for the outermost delimiters.
Once the prefix and the outermost opening delimiter bracket have been
recognized, code blocks are extracted by stepping through the input
text and trying the following alternatives in sequence:
1. Try and match a closing delimiter bracket. If the bracket was the
same species as the last opening bracket, return the substring to
that point. If the bracket was mismatched, return an error.
2. Try to match a quote or quotelike operator. If found, call
"extract_quotelike" to eat it. If "extract_quotelike" fails,
return the error it returned. Otherwise go back to step 1.
3. Try to match an opening delimiter bracket. If found, call
"extract_codeblock" recursively to eat the embedded block. If the
recursive call fails, return an error. Otherwise, go back to step
1.
4. Unconditionally match a bareword or any other single character,
and then go back to step 1.
Examples:
# Find a while loop in the text
if ($text =~ s/.*?while\s*\{/{/)
{
$loop = "while " . extract_codeblock($text);
}
# Remove the first round-bracketed list (which may include
# round- or curly-bracketed code blocks or quotelike operators)
extract_codeblock $text, "(){}", '[^(]*';
The ability to specify a different outermost delimiter bracket is
useful in some circumstances. For example, in the Parse::RecDescent
module, parser actions which are to be performed only on a successful
parse are specified using a "<defer:...>" directive. For example:
sentence: subject verb object
<defer: {$::theVerb = $item{verb}} >
Parse::RecDescent uses "extract_codeblock($text, '{}<>')" to extract
the code within the "<defer:...>" directive, but there's a problem.
A deferred action like this:
<defer: {if ($count>10) {$count--}} >
will be incorrectly parsed as:
<defer: {if ($count>
because the "less than" operator is interpreted as a closing
delimiter.
But, by extracting the directive using
"extract_codeblock($text, '{}', undef, '<>')" the '>' character is
only treated as a delimited at the outermost level of the code block,
so the directive is parsed correctly.
"extract_multiple"
The "extract_multiple" subroutine takes a string to be processed and
a list of extractors (subroutines or regular expressions) to apply to
that string.
In an array context "extract_multiple" returns an array of substrings
of the original string, as extracted by the specified extractors. In
a scalar context, "extract_multiple" returns the first substring
successfully extracted from the original string. In both scalar and
void contexts the original string has the first successfully
extracted substring removed from it. In all contexts
"extract_multiple" starts at the current "pos" of the string, and
sets that "pos" appropriately after it matches.
Hence, the aim of a call to "extract_multiple" in a list context is
to split the processed string into as many non-overlapping fields as
possible, by repeatedly applying each of the specified extractors to
the remainder of the string. Thus "extract_multiple" is a generalized
form of Perl's "split" subroutine.
The subroutine takes up to four optional arguments:
1. A string to be processed ($_ if the string is omitted or "undef")
2. A reference to a list of subroutine references and/or qr//
objects and/or literal strings and/or hash references, specifying
the extractors to be used to split the string. If this argument
is omitted (or "undef") the list:
[
sub { extract_variable($_[0], '') },
sub { extract_quotelike($_[0],'') },
sub { extract_codeblock($_[0],'{}','') },
]
is used.
3. An number specifying the maximum number of fields to return. If
this argument is omitted (or "undef"), split continues as long as
possible.
If the third argument is _N, then extraction continues until _N
fields have been successfully extracted, or until the string has
been completely processed.
Note that in scalar and void contexts the value of this argument
is automatically reset to 1 (under "-w", a warning is issued if
the argument has to be reset).
4. A value indicating whether unmatched substrings (see below)
within the text should be skipped or returned as fields. If the
value is true, such substrings are skipped. Otherwise, they are
returned.
The extraction process works by applying each extractor in sequence
to the text string.
If the extractor is a subroutine it is called in a list context and
is expected to return a list of a single element, namely the
extracted text. It may optionally also return two further arguments:
a string representing the text left after extraction (like $' for a
pattern match), and a string representing any prefix skipped before
the extraction (like $` in a pattern match). Note that this is
designed to facilitate the use of other Text::Balanced subroutines
with "extract_multiple". Note too that the value returned by an
extractor subroutine need not bear any relationship to the
corresponding substring of the original text (see examples below).
If the extractor is a precompiled regular expression or a string, it
is matched against the text in a scalar context with a leading '\G'
and the gc modifiers enabled. The extracted value is either $1 if
that variable is defined after the match, or else the complete match
(i.e. $&).
If the extractor is a hash reference, it must contain exactly one
element. The value of that element is one of the above extractor
types (subroutine reference, regular expression, or string). The key
of that element is the name of a class into which the successful
return value of the extractor will be blessed.
If an extractor returns a defined value, that value is immediately
treated as the next extracted field and pushed onto the list of
fields. If the extractor was specified in a hash reference, the
field is also blessed into the appropriate class,
If the extractor fails to match (in the case of a regex extractor),
or returns an empty list or an undefined value (in the case of a
subroutine extractor), it is assumed to have failed to extract. If
none of the extractor subroutines succeeds, then one character is
extracted from the start of the text and the extraction subroutines
reapplied. Characters which are thus removed are accumulated and
eventually become the next field (unless the fourth argument is true,
in which case they are discarded).
For example, the following extracts substrings that are valid Perl
variables:
@fields = extract_multiple($text,
[ sub { extract_variable($_[0]) } ],
undef, 1);
This example separates a text into fields which are quote delimited,
curly bracketed, and anything else. The delimited and bracketed parts
are also blessed to identify them (the "anything else" is unblessed):
@fields = extract_multiple($text,
[
{ Delim => sub { extract_delimited($_[0],q{'"}) } },
{ Brack => sub { extract_bracketed($_[0],'{}') } },
]);
This call extracts the next single substring that is a valid Perl
quotelike operator (and removes it from $text):
$quotelike = extract_multiple($text,
[
sub { extract_quotelike($_[0]) },
], undef, 1);
Finally, here is yet another way to do comma-separated value parsing:
@fields = extract_multiple($csv_text,
[
sub { extract_delimited($_[0],q{'"}) },
qr/([^,]+)(.*)/,
],
undef,1);
The list in the second argument means: _"_T_r_y _a_n_d _e_x_t_r_a_c_t _a _' _o_r _"
_d_e_l_i_m_i_t_e_d _s_t_r_i_n_g_, _o_t_h_e_r_w_i_s_e _e_x_t_r_a_c_t _a_n_y_t_h_i_n_g _u_p _t_o _a _c_o_m_m_a_._._._". The
undef third argument means: _"_._._._a_s _m_a_n_y _t_i_m_e_s _a_s _p_o_s_s_i_b_l_e_._._._", and
the true value in the fourth argument means _"_._._._d_i_s_c_a_r_d_i_n_g _a_n_y_t_h_i_n_g
_e_l_s_e _t_h_a_t _a_p_p_e_a_r_s _(_i_._e_. _t_h_e _c_o_m_m_a_s_)_".
If you wanted the commas preserved as separate fields (i.e. like
split does if your split pattern has capturing parentheses), you
would just make the last parameter undefined (or remove it).
"gen_delimited_pat"
The "gen_delimited_pat" subroutine takes a single (string) argument
and
> builds a Friedl-style optimized regex that matches a string
delimited by any one of the characters in the single argument. For
example:
gen_delimited_pat(q{'"})
returns the regex:
(?:\"(?:\\\"|(?!\").)*\"|\'(?:\\\'|(?!\').)*\')
Note that the specified delimiters are automatically quotemeta'd.
A typical use of "gen_delimited_pat" would be to build special
purpose tags for "extract_tagged". For example, to properly ignore
"empty" XML elements (which might contain quoted strings):
my $empty_tag = '<(' . gen_delimited_pat(q{'"}) . '|.)+/>';
extract_tagged($text, undef, undef, undef, {ignore => [$empty_tag]} );
"gen_delimited_pat" may also be called with an optional second
argument, which specifies the "escape" character(s) to be used for
each delimiter. For example to match a Pascal-style string (where '
is the delimiter and '' is a literal ' within the string):
gen_delimited_pat(q{'},q{'});
Different escape characters can be specified for different
delimiters. For example, to specify that '/' is the escape for
single quotes and '%' is the escape for double quotes:
gen_delimited_pat(q{'"},q{/%});
If more delimiters than escape chars are specified, the last escape
char is used for the remaining delimiters. If no escape char is
specified for a given specified delimiter, '\' is used.
"delimited_pat"
Note that "gen_delimited_pat" was previously called "delimited_pat".
That name may still be used, but is now deprecated.
DDIIAAGGNNOOSSTTIICCSS #
In a list context, all the functions return "(undef,$original_text)" on
failure. In a scalar context, failure is indicated by returning "undef"
(in this case the input text is not modified in any way).
In addition, on failure in _a_n_y context, the $@ variable is set.
Accessing "$@->{error}" returns one of the error diagnostics listed
below. Accessing "$@->{pos}" returns the offset into the original string
at which the error was detected (although not necessarily where it
occurred!) Printing $@ directly produces the error message, with the
offset appended. On success, the $@ variable is guaranteed to be
"undef".
The available diagnostics are:
"Did not find a suitable bracket: "%s""
The delimiter provided to "extract_bracketed" was not one of
'()[]<>{}'.
"Did not find prefix: /%s/"
A non-optional prefix was specified but wasn't found at the start of
the text.
"Did not find opening bracket after prefix: "%s""
"extract_bracketed" or "extract_codeblock" was expecting a particular
kind of bracket at the start of the text, and didn't find it.
"No quotelike operator found after prefix: "%s""
"extract_quotelike" didn't find one of the quotelike operators "q",
"qq", "qw", "qx", "s", "tr" or "y" at the start of the substring it
was extracting.
"Unmatched closing bracket: "%c""
"extract_bracketed", "extract_quotelike" or "extract_codeblock"
encountered a closing bracket where none was expected.
"Unmatched opening bracket(s): "%s""
"extract_bracketed", "extract_quotelike" or "extract_codeblock" ran
out of characters in the text before closing one or more levels of
nested brackets.
"Unmatched embedded quote (%s)"
"extract_bracketed" attempted to match an embedded quoted substring,
but failed to find a closing quote to match it.
"Did not find closing delimiter to match '%s'"
"extract_quotelike" was unable to find a closing delimiter to match
the one that opened the quote-like operation.
"Mismatched closing bracket: expected "%c" but found "%s""
"extract_bracketed", "extract_quotelike" or "extract_codeblock" found
a valid bracket delimiter, but it was the wrong species. This usually
indicates a nesting error, but may indicate incorrect quoting or
escaping.
"No block delimiter found after quotelike "%s""
"extract_quotelike" or "extract_codeblock" found one of the quotelike
operators "q", "qq", "qw", "qx", "s", "tr" or "y" without a suitable
block after it.
"Did not find leading dereferencer"
"extract_variable" was expecting one of '$', '@', or '%' at the start
of a variable, but didn't find any of them.
"Bad identifier after dereferencer"
"extract_variable" found a '$', '@', or '%' indicating a variable,
but that character was not followed by a legal Perl identifier.
"Did not find expected opening bracket at %s"
"extract_codeblock" failed to find any of the outermost opening
brackets that were specified.
"Improperly nested codeblock at %s"
A nested code block was found that started with a delimiter that was
specified as being only to be used as an outermost bracket.
"Missing second block for quotelike "%s""
"extract_codeblock" or "extract_quotelike" found one of the quotelike
operators "s", "tr" or "y" followed by only one block.
"No match found for opening bracket"
"extract_codeblock" failed to find a closing bracket to match the
outermost opening bracket.
"Did not find opening tag: /%s/"
"extract_tagged" did not find a suitable opening tag (after any
specified prefix was removed).
"Unable to construct closing tag to match: /%s/"
"extract_tagged" matched the specified opening tag and tried to
modify the matched text to produce a matching closing tag (because
none was specified). It failed to generate the closing tag, almost
certainly because the opening tag did not start with a bracket of
some kind.
"Found invalid nested tag: %s"
"extract_tagged" found a nested tag that appeared in the "reject"
list (and the failure mode was not "MAX" or "PARA").
"Found unbalanced nested tag: %s"
"extract_tagged" found a nested opening tag that was not matched by a
corresponding nested closing tag (and the failure mode was not "MAX"
or "PARA").
"Did not find closing tag"
"extract_tagged" reached the end of the text without finding a
closing tag to match the original opening tag (and the failure mode
was not "MAX" or "PARA").
EEXXPPOORRTTSS #
The following symbols are, or can be, exported by this module:
Default Exports
_N_o_n_e.
Optional Exports
"extract_delimited", "extract_bracketed", "extract_quotelike",
"extract_codeblock", "extract_variable", "extract_tagged",
"extract_multiple", "gen_delimited_pat", "gen_extract_tagged",
"delimited_pat".
Export Tags
“:ALL” #
"extract_delimited", "extract_bracketed", "extract_quotelike",
"extract_codeblock", "extract_variable", "extract_tagged",
"extract_multiple", "gen_delimited_pat", "gen_extract_tagged",
"delimited_pat".
KKNNOOWWNN BBUUGGSS #
See
<https://rt.cpan.org/Dist/Display.html?Status=Active&Queue=Text-Balanced>.
FFEEEEDDBBAACCKK #
Patches, bug reports, suggestions or any other feedback is welcome.
Patches can be sent as GitHub pull requests at
<https://github.com/steve-m-hay/Text-Balanced/pulls>.
Bug reports and suggestions can be made on the CPAN Request Tracker at
<https://rt.cpan.org/Public/Bug/Report.html?Queue=Text-Balanced>.
Currently active requests on the CPAN Request Tracker can be viewed at
<https://rt.cpan.org/Public/Dist/Display.html?Status=Active;Queue=Text-Balanced>.
Please test this distribution. See CPAN Testers Reports at
<https://www.cpantesters.org/> for details of how to get involved.
Previous test results on CPAN Testers Reports can be viewed at
<https://www.cpantesters.org/distro/T/Text-Balanced.html>.
Please rate this distribution on CPAN Ratings at
<https://cpanratings.perl.org/rate/?distribution=Text-Balanced>.
AAVVAAIILLAABBIILLIITTYY #
The latest version of this module is available from CPAN (see "CPAN" in
perlmodlib for details) at
<https://metacpan.org/release/Text-Balanced> or
<https://www.cpan.org/authors/id/S/SH/SHAY/> or
<https://www.cpan.org/modules/by-module/Text/>.
The latest source code is available from GitHub at
<https://github.com/steve-m-hay/Text-Balanced>.
IINNSSTTAALLLLAATTIIOONN #
See the _I_N_S_T_A_L_L file.
AAUUTTHHOORR #
Damian Conway <damian@conway.org <mailto:damian@conway.org>>.
Steve Hay <shay@cpan.org <mailto:shay@cpan.org>> is now maintaining
Text::Balanced as of version 2.03.
CCOOPPYYRRIIGGHHTT #
Copyright (C) 1997-2001 Damian Conway. All rights reserved.
Copyright (C) 2009 Adam Kennedy.
Copyright (C) 2015, 2020 Steve Hay. All rights reserved.
LLIICCEENNCCEE #
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, i.e. under the terms of either the
GNU General Public License or the Artistic License, as specified in the
_L_I_C_E_N_C_E file.
VVEERRSSIIOONN #
Version 2.04
DDAATTEE #
11 Dec 2020
HHIISSTTOORRYY #
See the _C_h_a_n_g_e_s file.
perl v5.36.3 2023-02-15 Text::Balanced(3p)