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. Unless stated otherwise 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
=item C<regnode_charclass>
-Character classes are represented by C<regnode_charclass> structures,
-which have a four-byte argument and then a 32-byte (256-bit) bitmap
-indicating which characters are included in the class.
+Bracketed character classes are represented by C<regnode_charclass>
+structures, which have a four-byte argument and then a 32-byte (256-bit)
+bitmap indicating which characters in the Latin1 range are included in
+the class.
regnode_charclass U32 arg1;
char bitmap[ANYOF_BITMAP_SIZE];
-=item C<regnode_charclass_class>
+Various flags whose names begin with C<ANYOF_> are used for special
+situations. Above Latin1 matches and things not known until run-time
+are stored in L</Perl's pprivate structure>.
+
+=item C<regnode_charclass_posixl>
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 classes
+POSIX char classes under C</l> matching,
+called C<regnode_charclass_posixl> which has an
+additional 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_posixl U32 arg1;
+ char bitmap[ANYOF_BITMAP_SIZE];
+ U32 classflags;
=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
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:
piece : _piece
| _piece quant
+=head3 Parsing complications
+
+The implication of the above description is that a pattern containing nested
+parentheses will result in a call graph which cycles through C<reg()>,
+C<regbranch()>, C<regpiece()>, C<regatom()>, C<reg()>, C<regbranch()> I<etc>
+multiple times, until the deepest level of nesting is reached. All the above
+routines return a pointer to a C<regnode>, which is usually the last regnode
+added to the program. However, one complication is that reg() returns NULL
+for parsing C<(?:)> syntax for embedded modifiers, setting the flag
+C<TRYAGAIN>. The C<TRYAGAIN> propagates upwards until it is captured, in
+some cases by C<regatom()>, but otherwise unconditionally by
+C<regbranch()>. Hence it will never be returned by C<regbranch()> to
+C<reg()>. This flag permits patterns such as C<(?i)+> to be detected as
+errors (I<Quantifier follows nothing in regex; marked by <-- HERE in m/(?i)+
+<-- HERE />).
+
+Another complication is that the representation used for the program differs
+if it needs to store Unicode, but it's not always possible to know for sure
+whether it does until midway through parsing. The Unicode representation for
+the program is larger, and cannot be matched as efficiently. (See L</Unicode
+and Localisation Support> below for more details as to why.) If the pattern
+contains literal Unicode, it's obvious that the program needs to store
+Unicode. Otherwise, the parser optimistically assumes that the more
+efficient representation can be used, and starts sizing on this basis.
+However, if it then encounters something in the pattern which must be stored
+as Unicode, such as an C<\x{...}> escape sequence representing a character
+literal, then this means that all previously calculated sizes need to be
+redone, using values appropriate for the Unicode representation. Currently,
+all regular expression constructions which can trigger this are parsed by code
+in C<regatom()>.
+
+To avoid wasted work when a restart is needed, the sizing pass is abandoned
+- C<regatom()> immediately returns NULL, setting the flag C<RESTART_UTF8>.
+(This action is encapsulated using the macro C<REQUIRE_UTF8>.) This restart
+request is propagated up the call chain in a similar fashion, until it is
+"caught" in C<Perl_re_op_compile()>, which marks the pattern as containing
+Unicode, and restarts the sizing pass. It is also possible for constructions
+within run-time code blocks to turn out to need Unicode representation.,
+which is signalled by C<S_compile_runtime_code()> returning false to
+C<Perl_re_op_compile()>.
+
+The restart was previously implemented using a C<longjmp> in C<regatom()>
+back to a C<setjmp> in C<Perl_re_op_compile()>, but this proved to be
+problematic as the latter is a large function containing many automatic
+variables, which interact badly with the emergent control flow of C<setjmp>.
+
=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)
and the second being running the regop interpreter.
If we can tell that there is no valid start point then we don't bother running
-interpreter at all. Likewise, if we know from the analysis phase that we
+the interpreter at all. Likewise, if we know from the analysis phase that we
cannot detect a short-cut to the start position, we go straight to the
interpreter.
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, 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.
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
UTF-8 properly, both at compile time and at execution time, including
when the string and pattern are mismatched.
-The following comment in F<regcomp.h> gives an example of exactly how
-tricky this can be:
-
- Two problematic code points in Unicode casefolding of EXACT nodes:
-
- U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
- U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
-
- which casefold to
-
- Unicode UTF-8
-
- U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
- U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
+=head2 Base Structures
- This means that in case-insensitive matching (or "loose matching",
- as Unicode calls it), an EXACTF of length six (the UTF-8 encoded
- byte length of the above casefolded versions) can match a target
- string of length two (the byte length of UTF-8 encoded U+0390 or
- U+03B0). This would rather mess up the minimum length computation.
+The C<regexp> structure described in L<perlreapi> is common to all
+regex engines. Two of its fields 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>.
- What we'll do is to look for the tail four bytes, and then peek
- at the preceding two bytes to see whether we need to decrease
- the minimum length by four (six minus two).
+Its C<pprivate> and C<intflags> fields contain data
+specific to each engine.
- Thanks to the design of UTF-8, there cannot be false matches:
- A sequence of valid UTF-8 bytes cannot be a subsequence of
- another valid sequence of UTF-8 bytes.
+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//>.
-=head2 Base Structures
-
-There are two structures used to store a compiled regular expression.
-One, the regexp structure, is considered to be perl's property, and the
-other is considered to be the property of the regex engine which
-compiled the regular expression; in the case of the stock engine this
-structure is called regexp_internal.
+The other structure is pointed to by 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
program and any additional data that is private to the regex engine
implementation.
-=head3 Perl Inspectable Data About Pattern
-
-F<regexp.h> contains the "public" structure definition. All regex engines
-must be able to correctly build a regexp structure.
-
- typedef struct regexp {
- /* what engine created this regexp? */
- const struct regexp_engine* engine;
-
- /* Information about the match that the perl core uses to manage things */
- U32 extflags; /* Flags used both externally and internally */
- I32 minlen; /* mininum possible length of string to match */
- I32 minlenret; /* mininum possible length of $& */
- U32 gofs; /* chars left of pos that we search from */
- struct reg_substr_data *substrs; /* substring data about strings that must appear
- in the final match, used for optimisations */
- U32 nparens; /* number of capture buffers */
-
- /* private engine specific data */
- U32 intflags; /* Engine Specific Internal flags */
- void *pprivate; /* Data private to the regex engine which
- created this object. */
-
- /* Data about the last/current match. These are modified during matching*/
- U32 lastparen; /* last open paren matched */
- U32 lastcloseparen; /* last close paren matched */
- I32 *startp; /* Array of offsets from start of string (@-) */
- I32 *endp; /* Array of offsets from start of string (@+) */
- char *subbeg; /* saved or original string
- so \digit works forever. */
- I32 sublen; /* Length of string pointed by subbeg */
- SV_SAVED_COPY /* If non-NULL, SV which is COW from original */
-
-
- /* Information about the match that isn't often used */
- char *precomp; /* pre-compilation regular expression */
- I32 prelen; /* length of precomp */
- I32 seen_evals; /* number of eval groups in the pattern - for security checks */
- HV *paren_names; /* Optional hash of paren names */
-
- /* Refcount of this regexp */
- I32 refcnt; /* Refcount of this regexp */
- } regexp;
-
-The fields are discussed in more detail below:
-
-=over 5
-
-
-=item C<refcnt>
-
-The number of times the structure is referenced. When this falls to 0
-the regexp is automatically freed by a call to pregfree.
-
-=item C<engine>
-
-This field points at a regexp_engine structure which contains pointers
-to the subroutines that are to be used for performing a match. It
-is the compiling routine's responsibility to populate this field before
-returning the regexp object.
-
-=item C<precomp> C<prelen>
-
-Used for debugging purposes. C<precomp> holds a copy of the pattern
-that was compiled.
-
-=item C<extflags>
-
-This is used to store various flags about the pattern, such as whether it
-contains a \G or a ^ or $ symbol.
-
-=item C<minlen> C<minlenret>
-
-C<minlen> is the minimum string length required for the pattern to match.
-This is used to prune the search space by not bothering to match any
-closer to the end of a string than would allow a match. For instance
-there is no point in even starting the regex engine if the minlen is
-10 but the string is only 5 characters long. There is no way that the
-pattern can match.
-
-C<minlenret> is the minimum length of the string that would be found
-in $& after a match.
-
-The difference between C<minlen> and C<minlenret> can be seen in the
-following pattern:
-
- /ns(?=\d)/
-
-where the C<minlen> would be 3 but the minlen ret would only be 2 as
-the \d is required to match but is not actually included in the matched
-content. This distinction is particularly important as the substitution
-logic uses the C<minlenret> to tell whether it can do in-place substition
-which can result in considerable speedup.
-
-=item C<gofs>
-
-Left offset from pos() to start match at.
-
-=item C<nparens>, C<lasparen>, and C<lastcloseparen>
-
-These fields are used to keep track of how many paren groups could be matched
-in the pattern, which was the last open paren to be entered, and which was
-the last close paren to be entered.
-
-=item C<paren_names>
-
-This is a hash used internally to track named capture buffers and their
-offsets. The keys are the names of the buffers the values are dualvars,
-with the IV slot holding the number of buffers with the given name and the
-pv being an embedded array of I32. The values may also be contained
-independently in the data array in cases where named backreferences are
-used.
-
-=item C<reg_substr_data>
-
-Holds information on the longest string that must occur at a fixed
-offset from the start of the pattern, and the longest string that must
-occur at a floating offset from the start of the pattern. Used to do
-Fast-Boyer-Moore searches on the string to find out if its worth using
-the regex engine at all, and if so where in the string to search.
-
-=item C<startp>, C<endp>
-
-These fields store arrays that are used to hold the offsets of the begining
-and end of each capture group that has matched. -1 is used to indicate no match.
-
-These are the source for @- and @+.
-
-=item C<subbeg> C<sublen> C<saved_copy>
-
-These are used during execution phase for managing search and replace
-patterns.
-
-=item C<seen_evals>
-
-This stores the number of eval groups in the pattern. This is used
-for security purposes when embedding compiled regexes into larger
-patterns.
-
-=back
-
-=head3 Engine Private Data About Pattern
-
-Additionally, regexp.h contains the following "private" definition which is
-perl-specific and 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;
+=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 {
+ 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 C<swap>
-
-C<swap> is an extra set of startp/endp stored in a C<regexp_paren_ofs>
-struct. This is used when the last successful match was from the same pattern
-as the current pattern, so that a partial match doesn't overwrite the
-previous match's results. When this field is data filled the matching
-engine will swap buffers before every match attempt. If the match fails,
-then it swaps them back. If it's successful it leaves them. This field
-is populated on demand and is by default null.
-
=item C<offsets>
Offsets holds a mapping of offset in the C<program>
=item C<data>
-This field points at a reg_data structure, which is defined as follows
+This field points at a C<reg_data> structure, which is defined as follows
struct reg_data {
U32 count;
=back
-=head2 Pluggable Interface
-
-As of Perl 5.9.5 there is a new interface for using other regexp engines
-than the default one. Each engine is supposed to provide access to
-a constant structure of the following format:
-
- typedef struct regexp_engine {
- regexp* (*comp) (pTHX_ char* exp, char* xend, U32 pm_flags);
- I32 (*exec) (pTHX_ regexp* prog, char* stringarg, char* strend,
- char* strbeg, I32 minend, SV* screamer,
- void* data, U32 flags);
- char* (*intuit) (pTHX_ regexp *prog, SV *sv, char *strpos,
- char *strend, U32 flags,
- struct re_scream_pos_data_s *data);
- SV* (*checkstr) (pTHX_ regexp *prog);
- void (*free) (pTHX_ struct regexp* r);
- #ifdef USE_ITHREADS
- void* (*dupe) (pTHX_ const regexp *r, CLONE_PARAMS *param);
- #endif
- } regexp_engine;
-
-When a regexp is compiled, its C<engine> field is then set to point at
-the appropriate structure so that when it needs to be used Perl can find
-the right routines to do so.
-
-In order to install a new regexp handler, C<$^H{regcomp}> is set
-to an integer which (when casted appropriately) resolves to one of these
-structures. When compiling, the C<comp> method is executed, and the
-resulting regexp structure's engine field is expected to point back at
-the same structure.
-
-The pTHX_ symbol in the definition is a macro used by perl under threading
-to provide an extra argument to the routine holding a pointer back to
-the interpreter that is executing the regexp. So under threading all
-routines get an extra argument.
-
-The routines are as follows:
-
-=over 4
-
-=item comp
-
- regexp* comp(char *exp, char *xend, U32 pm_flags);
-
-Compile the pattern between exp and xend using the flags contained in
-pm and return a pointer to a prepared regexp structure that can perform
-the match. pm flags will have the following flag bits set as determined
-by the context that comp() has been called from:
-
- RXf_UTF8 pattern is encoded in UTF8
- RXf_PMf_LOCALE use locale
- RXf_PMf_MULTILINE /m
- RXf_PMf_SINGLELINE /s
- RXf_PMf_FOLD /i
- RXf_PMf_EXTENDED /x
- RXf_PMf_KEEPCOPY /k
- RXf_SKIPWHITE split ' ' or split with no args
-
-In general these flags should be preserved in regex->extflags after
-compilation, although it is possible the regex includes constructs that
-changes them. The perl engine for instance may upgrade non-utf8 strings
-to utf8 if the pattern includes constructs such as C<\x{...}> that can only
-match unicode values. RXf_SKIPWHITE should always be preserved verbatim
-in regex->extflags.
-
-=item exec
-
- I32 exec(regexp* prog,
- char *stringarg, char* strend, char* strbeg,
- I32 minend, SV* screamer,
- void* data, U32 flags);
-
-Execute a regexp.
-
-=item intuit
-
- char* intuit( regexp *prog,
- SV *sv, char *strpos, char *strend,
- U32 flags, struct re_scream_pos_data_s *data);
-
-Find the start position where a regex match should be attempted,
-or possibly whether the regex engine should not be run because the
-pattern can't match. This is called as appropriate by the core
-depending on the values of the extflags member of the regexp
-structure.
-
-=item checkstr
-
- SV* checkstr(regexp *prog);
-
-Return a SV containing a string that must appear in the pattern. Used
-for optimising matches.
-
-=item free
-
- void free(regexp *prog);
-
-Called by perl when it is freeing a regexp pattern so that the engine
-can release any resources pointed to by the C<pprivate> member of the
-regexp structure. This is only responsible for freeing private data;
-perl will handle releasing anything else contained in the regexp structure.
-
-=item dupe
-
- void* dupe(const regexp *r, CLONE_PARAMS *param);
-
-On threaded builds a regexp may need to be duplicated so that the pattern
-can be used by mutiple threads. This routine is expected to handle the
-duplication of any private data pointed to by the C<pprivate> member of
-the regexp structure. It will be called with the preconstructed new
-regexp structure as an argument, the C<pprivate> member will point at
-the B<old> private structue, and it is this routine's responsibility to
-construct a copy and return a pointer to it (which perl will then use to
-overwrite the field as passed to this routine.)
-
-This allows the engine to dupe its private data but also if necessary
-modify the final structure if it really must.
-
-On unthreaded builds this field doesn't exist.
-
-=back
-
-
-=head2 De-allocation and Cloning
-
-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 regexp's data array based
-on the data item's type.
-
=head1 SEE ALSO
+L<perlreapi>
+
L<perlre>
L<perlunitut>
https://perl5.git.perl.org / perl5.git / blobdiff