regex engine, feedback on the perl5-porters mail list, and no doubt other
places as well.
-B<WARNING!> It should be clearly understood that this document
-represents the state of the regex engine as the author understands it at
-the time of writing. It is B<NOT> an API definition; it is purely an
-internals guide for those who want to hack the regex engine, or
-understand how the regex engine works. Readers of this document are
-expected to understand perl's regex syntax and its usage in detail. If
-you want to learn about the basics of Perl's regular expressions, see
-L<perlre>.
+B<NOTICE!> It should be clearly understood that the behavior and
+structures discussed in this represents the state of the engine as the
+author understood it at the time of writing. It is B<NOT> an API
+definition, it is purely an internals guide for those who want to hack
+the regex engine, or understand how the regex engine works. Readers of
+this document are expected to understand perl's regex syntax and its
+usage in detail. If you want to learn about the basics of Perl's
+regular expressions, see L<perlre>. And if you want to replace the
+regex engine with your own, see L<perlreapi>.
=head1 OVERVIEW
When speaking about regexes we need to distinguish between their source
code form and their internal form. In this document we will use the term
-"pattern" when we speak of their textual, source code form, the term
+"pattern" when we speak of their textual, source code form, and the term
"program" when we speak of their internal representation. These
correspond to the terms I<S-regex> and I<B-regex> that Mark Jason
-Dominus employs in his paper on "Rx" ([1] in L</references>).
+Dominus employs in his paper on "Rx" ([1] in L</REFERENCES>).
=head2 What is a regular expression engine?
target string, and determines whether or not the string satisfies the
constraints. See L<perlre> for a full definition of the language.
-So in less grandiose terms the first part of the job is to turn a pattern into
+In less grandiose terms, the first part of the job is to turn a pattern into
something the computer can efficiently use to find the matching point in
the string, and the second part is performing the search itself.
There is also a larger form of a char class structure used to represent
POSIX char classes called C<regnode_charclass_class> which has an
-additional 4-byte (32-bit) bitmap indicating which POSIX char class
+additional 4-byte (32-bit) bitmap indicating which POSIX char classes
have been included.
- regnode_charclass_class U32 arg1;
- char bitmap[ANYOF_BITMAP_SIZE];
- char classflags[ANYOF_CLASSBITMAP_SIZE];
+ regnode_charclass_class U32 arg1;
+ char bitmap[ANYOF_BITMAP_SIZE];
+ char classflags[ANYOF_CLASSBITMAP_SIZE];
=back
=item *
There is the "next regop" from a given regop/regnode. This is the
-regop physically located after the the current one, as determined by
+regop physically located after the current one, as determined by
the size of the current regop. This is often useful, such as when
dumping the structure we use this order to traverse. Sometimes the code
assumes that the "next regnode" is the same as the "next regop", or in
C<regbranch()> in turn calls C<regpiece()> which
handles "things" followed by a quantifier. In order to parse the
-"things", C<regatom()> is called. This is the lowest level routine which
+"things", C<regatom()> is called. This is the lowest level routine, which
parses out constant strings, character classes, and the
various special symbols like C<$>. If C<regatom()> encounters a "("
character it in turn calls C<reg()>.
-The routine C<regtail()> is called by both C<reg()>, C<regbranch()>
+The routine C<regtail()> is called by both C<reg()> and C<regbranch()>
in order to "set the tail pointer" correctly. When executing and
we get to the end of a branch, we need to go to the node following the
grouping parens. When parsing, however, we don't know where the end will
The call graph looks like this:
- reg() # parse a top level regex, or inside of parens
- regbranch() # parse a single branch of an alternation
- regpiece() # parse a pattern followed by a quantifier
- regatom() # parse a simple pattern
- regclass() # used to handle a class
- reg() # used to handle a parenthesised subpattern
- ....
- ...
- regtail() # finish off the branch
- ...
- regtail() # finish off the branch sequence. Tie each
- # branch's tail to the tail of the sequence
- # (NEW) In Debug mode this is
- # regtail_study().
+ reg() # parse a top level regex, or inside of
+ # parens
+ regbranch() # parse a single branch of an alternation
+ regpiece() # parse a pattern followed by a quantifier
+ regatom() # parse a simple pattern
+ regclass() # used to handle a class
+ reg() # used to handle a parenthesised
+ # subpattern
+ ....
+ ...
+ regtail() # finish off the branch
+ ...
+ regtail() # finish off the branch sequence. Tie each
+ # branch's tail to the tail of the
+ # sequence
+ # (NEW) In Debug mode this is
+ # regtail_study().
A grammar form might be something like this:
=head3 Debug Output
-In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE'; >> to see some trace
-information about the parse process. We will start with some simple
-patterns and build up to more complex patterns.
+In the 5.9.x development version of perl you can C<< use re Debug => 'PARSE' >>
+to see some trace information about the parse process. We will start with some
+simple patterns and build up to more complex patterns.
So when we parse C</foo/> we see something like the following table. The
left shows what is being parsed, and the number indicates where the next regop
atom
>)$< 34 tail~ BRANCH (28)
36 tsdy~ BRANCH (END) (31)
- ~ attach to CLOSE1 (34) offset to 3
+ ~ attach to CLOSE1 (34) offset to 3
tsdy~ EXACT <foo> (EXACT) (29)
- ~ attach to CLOSE1 (34) offset to 5
+ ~ attach to CLOSE1 (34) offset to 5
tsdy~ EXACT <bar> (EXACT) (32)
- ~ attach to CLOSE1 (34) offset to 2
+ ~ attach to CLOSE1 (34) offset to 2
>$< tail~ BRANCH (3)
~ BRANCH (9)
~ TAIL (25)
code that looks for C<\n> or the end of the string.
The next pointer for C<BRANCH>es is interesting in that it points at where
-execution should go if the branch fails. When executing if the engine
+execution should go if the branch fails. When executing, if the engine
tries to traverse from a branch to a C<regnext> that isn't a branch then
-the engine will know that the entire set of branches have failed.
+the engine will know that the entire set of branches has failed.
=head3 Peep-hole Optimisation and Analysis
=back
-Another form of optimisation that can occur is post-parse "peep-hole"
-optimisations, where inefficient constructs are replaced by
-more efficient constructs. An example of this are C<TAIL> regops which are used
-during parsing to mark the end of branches and the end of groups. These
-regops are used as place-holders during construction and "always match"
-so they can be "optimised away" by making the things that point to the
-C<TAIL> point to thing that the C<TAIL> points to, thus "skipping" the node.
+Another form of optimisation that can occur is the post-parse "peep-hole"
+optimisation, where inefficient constructs are replaced by more efficient
+constructs. The C<TAIL> regops which are used during parsing to mark the end
+of branches and the end of groups are examples of this. These regops are used
+as place-holders during construction and "always match" so they can be
+"optimised away" by making the things that point to the C<TAIL> point to the
+thing that C<TAIL> points to, thus "skipping" the node.
Another optimisation that can occur is that of "C<EXACT> merging" which is
where two consecutive C<EXACT> nodes are merged into a single
The two entry points are C<re_intuit_start()> and C<pregexec()>. These routines
have a somewhat incestuous relationship with overlap between their functions,
and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless
-other parts of the the perl source code may call into either, or both.
+other parts of the perl source code may call into either, or both.
-Execution of the interpreter itself used to be recursive. Due to the
-efforts of Dave Mitchell in the 5.9.x development track, it is now iterative. Now an
+Execution of the interpreter itself used to be recursive, but thanks to the
+efforts of Dave Mitchell in the 5.9.x development track, that has changed: now an
internal stack is maintained on the heap and the routine is fully
iterative. This can make it tricky as the code is quite conservative
-about what state it stores, with the result that that two consecutive lines in the
+about what state it stores, with the result that two consecutive lines in the
code can actually be running in totally different contexts due to the
simulated recursion.
It is basically a set-up wrapper around C<regmatch()>.
C<regmatch> is the main "recursive loop" of the interpreter. It is
-basically a giant switch statement that executes the regops based on
-their type. A few of the regops are implemented as subroutines but
-the bulk are inline code.
+basically a giant switch statement that implements a state machine, where
+the possible states are the regops themselves, plus a number of additional
+intermediate and failure states. A few of the states are implemented as
+subroutines but the bulk are inline code.
=head1 MISCELLANEOUS
=head2 Unicode and Localisation Support
When dealing with strings containing characters that cannot be represented
-using an eight-bit character set, perl uses an internal representation
+using an eight-bit character set, perl uses an internal representation
that is a permissive version of Unicode's UTF-8 encoding[2]. This uses single
-bytes to represent characters from the ASCII character set, and sequences
+bytes to represent characters from the ASCII character set, and sequences
of two or more bytes for all other characters. (See L<perlunitut>
for more information about the relationship between UTF-8 and perl's
-encoding, utf8 -- the difference isn't important for this discussion.)
+encoding, utf8. The difference isn't important for this discussion.)
No matter how you look at it, Unicode support is going to be a pain in a
regex engine. Tricks that might be fine when you have 256 possible
C<sizeof(char1) == sizeof(char2)>, but in UTF-8 it isn't. Unicode case folding is
vastly more complex than the simple rules of ASCII, and even when not
using Unicode but only localised single byte encodings, things can get
-tricky (for example, GERMAN-SHARP-ESS should match 'SS' in localised
-case-insensitive matching).
+tricky (for example, B<LATIN SMALL LETTER SHARP S> (U+00DF, E<szlig>)
+should match 'SS' in localised case-insensitive matching).
Making things worse is that UTF-8 support was a later addition to the
regex engine (as it was to perl) and this necessarily made things a lot
A sequence of valid UTF-8 bytes cannot be a subsequence of
another valid sequence of UTF-8 bytes.
-=head2 Base Struct
-
-F<regexp.h> contains the base structure definition:
-
- typedef struct regexp {
- I32 *startp;
- I32 *endp;
- regnode *regstclass;
- struct reg_substr_data *substrs;
- char *precomp; /* pre-compilation regular expression */
- struct reg_data *data; /* Additional data. */
- char *subbeg; /* saved or original string
- so \digit works forever. */
- #ifdef PERL_OLD_COPY_ON_WRITE
- SV *saved_copy; /* If non-NULL, SV which is COW from original */
- #endif
- U32 *offsets; /* offset annotations 20001228 MJD */
- I32 sublen; /* Length of string pointed by subbeg */
- I32 refcnt;
- I32 minlen; /* minimum possible length of $& */
- I32 prelen; /* length of precomp */
- U32 nparens; /* number of parentheses */
- U32 lastparen; /* last paren matched */
- U32 lastcloseparen; /* last paren matched */
- U32 reganch; /* Internal use only +
- Tainted information used by regexec? */
- regnode program[1]; /* Unwarranted chumminess with compiler. */
- } regexp;
-
-C<program>, and C<data> are the primary fields of concern in terms of
-program structure. C<program> is the actual array of nodes, and C<data> is
-an array of "whatever", with each whatever being typed by letter, and
-freed or cloned as needed based on this type. regops use the data
-array to store reference data that isn't convenient to store in the regop
-itself. It also means memory management code doesn't need to traverse the
-program to find pointers. So for instance, if a regop needs a pointer, the
-normal procedure is use a C<regnode_arg1> store the data index in the C<ARG>
-field and look it up from the data array.
+
+=head2 Base Structures
+
+The C<regexp> structure described in L<perlreapi> is common to all
+regex engines. Two of its fields that are intended for the private use
+of the regex engine that compiled the pattern. These are the
+C<intflags> and pprivate members. The C<pprivate> is a void pointer to
+an arbitrary structure whose use and management is the responsibility
+of the compiling engine. perl will never modify either of these
+values. In the case of the stock engine the structure pointed to by
+C<pprivate> is called C<regexp_internal>.
+
+Its C<pprivate> and C<intflags> fields contain data
+specific to each engine.
+
+There are two structures used to store a compiled regular expression.
+One, the C<regexp> structure described in L<perlreapi> is populated by
+the engine currently being. used and some of its fields read by perl to
+implement things such as the stringification of C<qr//>.
+
+
+The other structure is pointed to be the C<regexp> struct's
+C<pprivate> and is in addition to C<intflags> in the same struct
+considered to be the property of the regex engine which compiled the
+regular expression;
+
+The regexp structure contains all the data that perl needs to be aware of
+to properly work with the regular expression. It includes data about
+optimisations that perl can use to determine if the regex engine should
+really be used, and various other control info that is needed to properly
+execute patterns in various contexts such as is the pattern anchored in
+some way, or what flags were used during the compile, or whether the
+program contains special constructs that perl needs to be aware of.
+
+In addition it contains two fields that are intended for the private use
+of the regex engine that compiled the pattern. These are the C<intflags>
+and pprivate members. The C<pprivate> is a void pointer to an arbitrary
+structure whose use and management is the responsibility of the compiling
+engine. perl will never modify either of these values.
+
+As mentioned earlier, in the case of the default engines, the C<pprivate>
+will be a pointer to a regexp_internal structure which holds the compiled
+program and any additional data that is private to the regex engine
+implementation.
+
+=head3 Perl's C<pprivate> structure
+
+The following structure is used as the C<pprivate> struct by perl's
+regex engine. Since it is specific to perl it is only of curiosity
+value to other engine implementations.
+
+ typedef struct regexp_internal {
+ regexp_paren_ofs *swap; /* Swap copy of *startp / *endp */
+ U32 *offsets; /* offset annotations 20001228 MJD
+ * data about mapping the program to
+ * the string*/
+ regnode *regstclass; /* Optional startclass as identified or
+ * constructed by the optimiser */
+ struct reg_data *data; /* Additional miscellaneous data used
+ * by the program. Used to make it
+ * easier to clone and free arbitrary
+ * data that the regops need. Often the
+ * ARG field of a regop is an index
+ * into this structure */
+ regnode program[1]; /* Unwarranted chumminess with
+ * compiler. */
+ } regexp_internal;
=over 5
-=item -
+=item C<swap>
-C<startp>, C<endp>, C<nparens>, C<lasparen>, and C<lastcloseparen> are used to manage capture
-buffers.
+C<swap> formerly was an extra set of startp/endp stored in a
+C<regexp_paren_ofs> struct. This was used when the last successful match
+was from the same pattern as the current pattern, so that a partial
+match didn't overwrite the previous match's results, but it caused a
+problem with re-entrant code such as trying to build the UTF-8 swashes.
+Currently unused and left for backward compatibility with 5.10.0.
-=item -
+=item C<offsets>
-C<subbeg> and optional C<saved_copy> are used during the execution phase for managing
-replacements.
+Offsets holds a mapping of offset in the C<program>
+to offset in the C<precomp> string. This is only used by ActiveState's
+visual regex debugger.
-=item -
+=item C<regstclass>
-C<offsets> and C<precomp> are used for debugging purposes.
+Special regop that is used by C<re_intuit_start()> to check if a pattern
+can match at a certain position. For instance if the regex engine knows
+that the pattern must start with a 'Z' then it can scan the string until
+it finds one and then launch the regex engine from there. The routine
+that handles this is called C<find_by_class()>. Sometimes this field
+points at a regop embedded in the program, and sometimes it points at
+an independent synthetic regop that has been constructed by the optimiser.
-=item -
+=item C<data>
-The rest are used for start point optimisations.
+This field points at a reg_data structure, which is defined as follows
-=back
+ struct reg_data {
+ U32 count;
+ U8 *what;
+ void* data[1];
+ };
-=head2 De-allocation and Cloning
+This structure is used for handling data structures that the regex engine
+needs to handle specially during a clone or free operation on the compiled
+product. Each element in the data array has a corresponding element in the
+what array. During compilation regops that need special structures stored
+will add an element to each array using the add_data() routine and then store
+the index in the regop.
-Any patch that adds data items to the regexp will need to include
-changes to F<sv.c> (C<Perl_re_dup()>) and F<regcomp.c> (C<pregfree()>). This
-involves freeing or cloning items in the regexes data array based
-on the data item's type.
+=item C<program>
+
+Compiled program. Inlined into the structure so the entire struct can be
+treated as a single blob.
+
+=back
=head1 SEE ALSO
+L<perlreapi>
+
L<perlre>
L<perlunitut>
https://perl5.git.perl.org / perl5.git / blobdiff