=head1 DESCRIPTION
This document is an attempt to shine some light on the guts of the regex
-engine and how it works. The regex engine represents a signifigant chunk
+engine and how it works. The regex engine represents a significant chunk
of the perl codebase, but is relatively poorly understood. This document
is a meagre attempt at addressing this situation. It is derived from the
-authors experience, comments in the source code, other papers on the
-regex engine, feedback in p5p, 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 perls regex syntax and its usage in detail, if
-you are a beginner you are in the wrong the place.
+author's experience, comments in the source code, other papers on the
+regex engine, feedback on the perl5-porters mail list, and no doubt other
+places as well.
+
+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
=head2 A quick note on terms
-There is some debate as to whether to say 'regexp' or 'regex'. In this
+There is some debate as to whether to say "regexp" or "regex". In this
document we will use the term "regex" unless there is a special reason
-not to, and then we will explain why.
+not to, in which case we will explain why.
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 C<S-regex> and C<B-regex> that Mark Jason
-Dominus employs in his paper on "Rx"[1].
+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>).
=head2 What is a regular expression engine?
-A regular expression engine is a program whose job is to efficiently
-find a section of a string that matches a set criteria of criteria. The
-criteria is expressed in text using a formal language. See perlre for a
-full definition of the language.
+A regular expression engine is a program that takes a set of constraints
+specified in a mini-language, and then applies those constraints to a
+target string, and determines whether or not the string satisfies the
+constraints. See L<perlre> for a full definition of the language.
-So the job in less grandiose terms is to some 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.
+the string, and the second part is performing the search itself.
To do this we need to produce a program by parsing the text. We then
need to execute the program to find the point in the string that
=head3 High Level
-Although it is a bit confusing and some object to the terminology it
+Although it is a bit confusing and some people object to the terminology, it
is worth taking a look at a comment that has
-been in regexp.h for years:
+been in F<regexp.h> for years:
I<This is essentially a linear encoding of a nondeterministic
finite-state machine (aka syntax charts or "railroad normal form" in
The term "railroad normal form" is a bit esoteric, with "syntax
diagram/charts", or "railroad diagram/charts" being more common terms.
-Nevertheless it provides a useful mental image of a regex program: Each
+Nevertheless it provides a useful mental image of a regex program: each
node can be thought of as a unit of track, with a single entry and in
most cases a single exit point (there are pieces of track that fork, but
-statistically not many), and the total forms a system of track with a
+statistically not many), and the whole forms a layout with a
single entry and single exit point. The matching process can be thought
-of as a car that moves on the track, with the particular route through
+of as a car that moves along the track, with the particular route through
the system being determined by the character read at each possible
-connector point. A car can roll off the track at any point but it may
-not procede unless it matches the track...
+connector point. A car can fall off the track at any point but it may
+only proceed as long as it matches the track.
Thus the pattern C</foo(?:\w+|\d+|\s+)bar/> can be thought of as the
following chart:
- [start]
- |
- <foo>
- |
- +---+---+
- | | |
- <\w+> | <\s+>
- | <\d+> |
- | | |
- +---+---+
- |
- <bar>
- |
- [end]
-
-The truth of the matter is that perls regular expressions these days are
-way beyond such models, but they can help when trying to get your
-bearings, and they do match pretty closely with the current
-implementation.
-
-To be more precise we will say that a regex program is an encoding
-of a graph. Each node in the graph corresponds to part of
+ [start]
+ |
+ <foo>
+ |
+ +-----+-----+
+ | | |
+ <\w+> <\d+> <\s+>
+ | | |
+ +-----+-----+
+ |
+ <bar>
+ |
+ [end]
+
+The truth of the matter is that perl's regular expressions these days are
+much more complex than this kind of structure, but visualising it this way
+can help when trying to get your bearings, and it matches the
+current implementation pretty closely.
+
+To be more precise, we will say that a regex program is an encoding
+of a graph. Each node in the graph corresponds to part of
the original regex pattern, such as a literal string or a branch,
and has a pointer to the nodes representing the next component
-to be matched. Since "node" and opcode are overloaded terms in the
-perl source, we will call the nodes in a regex program 'regops'.
+to be matched. Since "node" and "opcode" already have other meanings in the
+perl source, we will call the nodes in a regex program "regops".
-The program is represented by an array of regnode structures, one or
-more of which together represent a single regop of the program. Struct
-regnode is the smallest struct needed and has a field structure which is
+The program is represented by an array of C<regnode> structures, one or
+more of which represent a single regop of the program. Struct
+C<regnode> is the smallest struct needed, and has a field structure which is
shared with all the other larger structures.
-"Next" pointers of all regops except BRANCH implement concatenation; a
-"next" pointer with a BRANCH on both ends of it is connecting two
-alternatives. [Here we have one of the subtle syntax dependencies: an
-individual BRANCH (as opposed to a collection of them) is never
-concatenated with anything because of operator precedence.
+The "next" pointers of all regops except C<BRANCH> implement concatenation;
+a "next" pointer with a C<BRANCH> on both ends of it is connecting two
+alternatives. [Here we have one of the subtle syntax dependencies: an
+individual C<BRANCH> (as opposed to a collection of them) is never
+concatenated with anything because of operator precedence.]
The operand of some types of regop is a literal string; for others,
it is a regop leading into a sub-program. In particular, the operand
-of a BRANCH node is the first regop of the branch.
+of a C<BRANCH> node is the first regop of the branch.
-B<NOTE>: As the railroad metaphor suggests this is B<not> a tree
+B<NOTE>: As the railroad metaphor suggests, this is B<not> a tree
structure: the tail of the branch connects to the thing following the
-set of BRANCHes. It is a like a single line of railway track that
+set of C<BRANCH>es. It is a like a single line of railway track that
splits as it goes into a station or railway yard and rejoins as it comes
out the other side.
=head3 Regops
-The base structure of a regop is defined in regexp.h as follows:
+The base structure of a regop is defined in F<regexp.h> as follows:
struct regnode {
- U8 flags; /* Various purposes, sometimes overriden */
+ U8 flags; /* Various purposes, sometimes overridden */
U8 type; /* Opcode value as specified by regnodes.h */
U16 next_off; /* Offset in size regnode */
};
-Other larger regnode-like structures are defined in regcomp.h. They
+Other larger C<regnode>-like structures are defined in F<regcomp.h>. They
are almost like subclasses in that they have the same fields as
-regnode, with possibly additional fields following in
+C<regnode>, with possibly additional fields following in
the structure, and in some cases the specific meaning (and name)
-of some of base fields are overriden. The following is a more
+of some of base fields are overridden. The following is a more
complete description.
=over 4
-=item regnode_1
+=item C<regnode_1>
-=item regnode_2
+=item C<regnode_2>
-regnode_1 structures have the same header, followed by a single
-four-byte argument; regnode_2 structures contain two two-byte
+C<regnode_1> structures have the same header, followed by a single
+four-byte argument; C<regnode_2> structures contain two two-byte
arguments instead:
regnode_1 U32 arg1;
regnode_2 U16 arg1; U16 arg2;
-=item regnode_string
+=item C<regnode_string>
-regnode_string structures, used for literal strings, follow the header
+C<regnode_string> structures, used for literal strings, follow the header
with a one-byte length and then the string data. Strings are padded on
the end with zero bytes so that the total length of the node is a
multiple of four bytes:
regnode_string char string[1];
- U8 str_len; (overides flags)
+ U8 str_len; /* overrides flags */
-=item regnode_charclass
+=item C<regnode_charclass>
-character classes are represented by 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 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 regnode_charclass_class which contains the
-same fields plus an additional 4-byte (32-bit) bitmap indicating which
-POSIX char class have been included.
+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
-regnodes.h defines an array called regarglen[] which gives the size
-of each opcode in units of size regnode (4-byte). A macro is used
-to calculate the size of an EXACT node based on its C<str_len> field.
+F<regnodes.h> defines an array called C<regarglen[]> which gives the size
+of each opcode in units of C<size regnode> (4-byte). A macro is used
+to calculate the size of an C<EXACT> node based on its C<str_len> field.
-The opcodes are defined in regnodes.h which is generated from
-regcomp.sym by regcomp.pl. Currently the maximum possible number
-of distinct opcodes is restricted to 256, with about 1/4 already
+The regops are defined in F<regnodes.h> which is generated from
+F<regcomp.sym> by F<regcomp.pl>. Currently the maximum possible number
+of distinct regops is restricted to 256, with about a quarter already
used.
-There's a set of macros provided to make accessing the fields
-easier and more consistent. These include C<OP()> which is used to tell
-the type of a regnode-like structure, NEXT_OFF() which is the offset to
-the next node (more on this later), ARG(), ARG1(), ARG2(), ARG_SET(),
-and equivelents for reading and setting the arguments, STR_LEN(),
-STRING(), and OPERAND() for manipulating strings and regop bearing
+A set of macros makes accessing the fields
+easier and more consistent. These include C<OP()>, which is used to determine
+the type of a C<regnode>-like structure; C<NEXT_OFF()>, which is the offset to
+the next node (more on this later); C<ARG()>, C<ARG1()>, C<ARG2()>, C<ARG_SET()>,
+and equivalents for reading and setting the arguments; and C<STR_LEN()>,
+C<STRING()> and C<OPERAND()> for manipulating strings and regop bearing
types.
-=head3 What opcode is next?
+=head3 What regop is next?
There are three distinct concepts of "next" in the regex engine, and
it is important to keep them clear.
=item *
-There is the "next opcode" from a given opcode/regnode. This is the
-opcode physically located after the the current one, as determined by
-the size of the current opcode. This is often useful, such as when
+There is the "next regop" from a given regop/regnode. This is the
+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 opcode", or in
-other words assumes that the sizeof a given opcode type is always going
-to be 1 regnode large.
+assumes that the "next regnode" is the same as the "next regop", or in
+other words assumes that the sizeof a given regop type is always going
+to be one regnode large.
=item *
-There is the "regnext" from a given opcode. This is the opcode which
-is reached by jumping forward by the value of NEXT_OFF(),
-or in a few cases for longer jumps by the arg1 field of the regnode_1
-structure. The subroutine regnext() handles this transparently.
+There is the "regnext" from a given regop. This is the regop which
+is reached by jumping forward by the value of C<NEXT_OFF()>,
+or in a few cases for longer jumps by the C<arg1> field of the C<regnode_1>
+structure. The subroutine C<regnext()> handles this transparently.
This is the logical successor of the node, which in some cases, like
-that of the BRANCH opcode, has special meaning.
+that of the C<BRANCH> regop, has special meaning.
+
+=back
+
+=head1 Process Overview
+
+Broadly speaking, performing a match of a string against a pattern
+involves the following steps:
+
+=over 5
+
+=item A. Compilation
+
+=over 5
+
+=item 1. Parsing for size
+
+=item 2. Parsing for construction
+
+=item 3. Peep-hole optimisation and analysis
=back
-=head1 PROCESS OVERVIEW
+=item B. Execution
+
+=over 5
-Broadly speaking performing a match of a string against a pattern
-involves the following steps
+=item 4. Start position and no-match optimisations
+
+=item 5. Program execution
+
+=back
+
+=back
- A. Compilation
- 1. Parsing for size
- 2. Parsing for construction
- 3. Peep-hole Optimisation and Analysis
- B. Execution
- 4. Start position and no-match optimisations
- 5. Program execution
Where these steps occur in the actual execution of a perl program is
determined by whether the pattern involves interpolating any string
-variables. If it does then compilation happens at run time. If it
-doesn't then it happens at compile time. (The C</o> modifier changes this,
+variables. If interpolation occurs, then compilation happens at run time. If it
+does not, then compilation is performed at compile time. (The C</o> modifier changes this,
as does C<qr//> to a certain extent.) The engine doesn't really care that
much.
=head2 Compilation
-This code exists primarily in regcomp.c, along with the header files
-regcomp.h, regexp.h, regnodes.h.
+This code resides primarily in F<regcomp.c>, along with the header files
+F<regcomp.h>, F<regexp.h> and F<regnodes.h>.
-Compilation starts with C<pregcomp()>, which is mostly an initialization
-wrapper which farms out two other routines for the heavy lifting. The
-first being C<reg()> which is the start point for parsing, and
-C<study_chunk()> which is responsible for optimisation.
+Compilation starts with C<pregcomp()>, which is mostly an initialisation
+wrapper which farms work out to two other routines for the heavy lifting: the
+first is C<reg()>, which is the start point for parsing; the second,
+C<study_chunk()>, is responsible for optimisation.
-Initialization in C<pregcomp()> mostly involves the creation and data
-filling of a special structure RExC_state_t, (defined in regcomp.c).
-Almost all internally used routines in regcomp.h take a pointer to one
-of these structures as their first argument, with the name *pRExC_state.
+Initialisation in C<pregcomp()> mostly involves the creation and data-filling
+of a special structure, C<RExC_state_t> (defined in F<regcomp.c>).
+Almost all internally-used routines in F<regcomp.h> take a pointer to one
+of these structures as their first argument, with the name C<pRExC_state>.
This structure is used to store the compilation state and contains many
-fields. Likewise their are many macros defined which operate on this
-variable. Anything that looks like RExC_xxxx is a macro that operates on
+fields. Likewise there are many macros which operate on this
+variable: anything that looks like C<RExC_xxxx> is a macro that operates on
this pointer/structure.
=head3 Parsing for size
In this pass the input pattern is parsed in order to calculate how much
-space is needed for each opcode we would need to emit. The size is also
+space is needed for each regop we would need to emit. The size is also
used to determine whether long jumps will be required in the program.
-This stage is controlled by the macro SIZE_ONLY being set.
+This stage is controlled by the macro C<SIZE_ONLY> being set.
-The parse procedes pretty much exactly as it does during the
-construction phase except that most routines are shortcircuited to
-change the size field RExC_size and not actually do anything.
+The parse proceeds pretty much exactly as it does during the
+construction phase, except that most routines are short-circuited to
+change the size field C<RExC_size> and not do anything else.
-=head3 Parsing for construcution
+=head3 Parsing for construction
-Once the size of the program has been determine the pattern is parsed
-again, but this time for real. Now SIZE_ONLY will be false, and the
+Once the size of the program has been determined, the pattern is parsed
+again, but this time for real. Now C<SIZE_ONLY> will be false, and the
actual construction can occur.
C<reg()> is the start of the parse process. It is responsible for
parsing an arbitrary chunk of pattern up to either the end of the
string, or the first closing parenthesis it encounters in the pattern.
-This means it can be used to parse the toplevel regex, or any section
+This means it can be used to parse the top-level regex, or any section
inside of a grouping parenthesis. It also handles the "special parens"
-that perls regexes have. For instance when parsing C</x(?:foo)y/> C<reg()>
-will at one point be called to parse from the '?' symbol up to and
-including the ')'.
+that perl's regexes have. For instance when parsing C</x(?:foo)y/> C<reg()>
+will at one point be called to parse from the "?" symbol up to and
+including the ")".
-Additionally C<reg()> is responsible for parsing the one or more
+Additionally, C<reg()> is responsible for parsing the one or more
branches from the pattern, and for "finishing them off" by correctly
-setting their next pointers. In order to do the parsing it repeatedly
-calls out to C<regbranch()> which is responsible for handling up to the
+setting their next pointers. In order to do the parsing, it repeatedly
+calls out to C<regbranch()>, which is responsible for handling up to the
first C<|> symbol it sees.
-C<regbranch()> in turn calls C<regpiece()> which is responsible for
-handling "things" followed by a quantifier. In order to parse the
-"things" C<regatom()> is called. This is the lowest level routine which
-is responsible for parsing out constant strings, char classes, and the
-various special symbols like C<$>. If C<regatom()> encounters a '('
+C<regbranch()> in turn calls C<regpiece()> which
+handles "things" followed by a quantifier. In order to parse the
+"things", C<regatom()> is called. This is the lowest level routine, which
+parses out constant strings, character classes, and the
+various special symbols like C<$>. 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 node following the
-grouping parens. When parsing however we don't know where the end will
+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
be until we get there, so when we do we must go back and update the
offsets as appropriate. C<regtail> is used to make this easier.
-A subtlety of the parse process means that a regex like C</foo/> is
+A subtlety of the parsing process means that a regex like C</foo/> is
originally parsed into an alternation with a single branch. It is only
-afterwards that the optimizer converts single branch alternations into the
+afterwards that the optimiser converts single branch alternations into the
simpler form.
=head3 Parse Call Graph and a Grammar
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 parenthesized subpattern
- ....
- ...
- regtail() # finish off the branch
- ...
- regtail() # finish off the branch sequence. Tie each
- # branches 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 bleadperl 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 whats being parsed, the number indicates where the next regop
+left shows what is being parsed, and the number indicates where the next regop
would go. The stuff on the right is the trace output of the graph. The
names are chosen to be short to make it less dense on the screen. 'tsdy'
is a special form of C<regtail()> which does some extra analysis.
- >foo< 1 reg
- brnc
- piec
- atom
- >< 4 tsdy~ EXACT <foo> (EXACT) (1)
- ~ attach to END (3) offset to 2
+ >foo< 1 reg
+ brnc
+ piec
+ atom
+ >< 4 tsdy~ EXACT <foo> (EXACT) (1)
+ ~ attach to END (3) offset to 2
The resulting program then looks like:
3: END(0)
As you can see, even though we parsed out a branch and a piece, it was ultimately
-only an atom. The final program shows us how things work. We have an EXACT regop,
-followed by an END regop. The number in parens indicates where the 'regnext' of
-the node goes. The 'regnext' of an END regop is unused, as END regops mean
+only an atom. The final program shows us how things work. We have an C<EXACT> regop,
+followed by an C<END> regop. The number in parens indicates where the C<regnext> of
+the node goes. The C<regnext> of an C<END> regop is unused, as C<END> regops mean
we have successfully matched. The number on the left indicates the position of
the regop in the regnode array.
-Now lets try a harder pattern. We will add a quantifier so we have the pattern
-C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> regpiece twice.
+Now let's try a harder pattern. We will add a quantifier, so now we have the pattern
+C</foo+/>. We will see that C<regbranch()> calls C<regpiece()> twice.
- >foo+< 1 reg
- brnc
- piec
- atom
- >o+< 3 piec
- atom
- >< 6 tail~ EXACT <fo> (1)
- 7 tsdy~ EXACT <fo> (EXACT) (1)
- ~ PLUS (END) (3)
- ~ attach to END (6) offset to 3
+ >foo+< 1 reg
+ brnc
+ piec
+ atom
+ >o+< 3 piec
+ atom
+ >< 6 tail~ EXACT <fo> (1)
+ 7 tsdy~ EXACT <fo> (EXACT) (1)
+ ~ PLUS (END) (3)
+ ~ attach to END (6) offset to 3
And we end up with the program:
4: EXACT <o>(0)
6: END(0)
-Now we have a special case. The EXACT regop has a regnext of 0. This is
-because if it matches it should try to match itself again. The PLUS regop
-handles the actual failure of the EXACT regop and acts appropriately (going
-to regnode 6 if the EXACT matched at least once, or failing if it didn't.)
+Now we have a special case. The C<EXACT> regop has a C<regnext> of 0. This is
+because if it matches it should try to match itself again. The C<PLUS> regop
+handles the actual failure of the C<EXACT> regop and acts appropriately (going
+to regnode 6 if the C<EXACT> matched at least once, or failing if it didn't).
Now for something much more complex: C</x(?:foo*|b[a][rR])(foo|bar)$/>
- >x(?:foo*|b... 1 reg
- brnc
+ >x(?:foo*|b... 1 reg
+ brnc
+ piec
+ atom
+ >(?:foo*|b[... 3 piec
+ atom
+ >?:foo*|b[a... reg
+ >foo*|b[a][... brnc
piec
atom
- >(?:foo*|b[... 3 piec
+ >o*|b[a][rR... 5 piec
+ atom
+ >|b[a][rR])... 8 tail~ EXACT <fo> (3)
+ >b[a][rR])(... 9 brnc
+ 10 piec
atom
- >?:foo*|b[a... reg
- >foo*|b[a][... brnc
- piec
- atom
- >o*|b[a][rR... 5 piec
- atom
- >|b[a][rR])... 8 tail~ EXACT <fo> (3)
- >b[a][rR])(... 9 brnc
- 10 piec
- atom
- >[a][rR])(f... 12 piec
- atom
- >a][rR])(fo... clas
- >[rR])(foo|... 14 tail~ EXACT <b> (10)
- piec
- atom
- >rR])(foo|b... clas
- >)(foo|bar)... 25 tail~ EXACT <a> (12)
- tail~ BRANCH (3)
- 26 tsdy~ BRANCH (END) (9)
- ~ attach to TAIL (25) offset to 16
- tsdy~ EXACT <fo> (EXACT) (4)
- ~ STAR (END) (6)
- ~ attach to TAIL (25) offset to 19
- tsdy~ EXACT <b> (EXACT) (10)
- ~ EXACT <a> (EXACT) (12)
- ~ ANYOF[Rr] (END) (14)
- ~ attach to TAIL (25) offset to 11
- >(foo|bar)$< tail~ EXACT <x> (1)
+ >[a][rR])(f... 12 piec
+ atom
+ >a][rR])(fo... clas
+ >[rR])(foo|... 14 tail~ EXACT <b> (10)
piec
atom
- >foo|bar)$< reg
- 28 brnc
- piec
- atom
- >|bar)$< 31 tail~ OPEN1 (26)
- >bar)$< brnc
- 32 piec
- atom
- >)$< 34 tail~ BRANCH (28)
- 36 tsdy~ BRANCH (END) (31)
- ~ attach to CLOSE1 (34) offset to 3
- tsdy~ EXACT <foo> (EXACT) (29)
- ~ attach to CLOSE1 (34) offset to 5
- tsdy~ EXACT <bar> (EXACT) (32)
- ~ attach to CLOSE1 (34) offset to 2
- >$< tail~ BRANCH (3)
- ~ BRANCH (9)
- ~ TAIL (25)
+ >rR])(foo|b... clas
+ >)(foo|bar)... 25 tail~ EXACT <a> (12)
+ tail~ BRANCH (3)
+ 26 tsdy~ BRANCH (END) (9)
+ ~ attach to TAIL (25) offset to 16
+ tsdy~ EXACT <fo> (EXACT) (4)
+ ~ STAR (END) (6)
+ ~ attach to TAIL (25) offset to 19
+ tsdy~ EXACT <b> (EXACT) (10)
+ ~ EXACT <a> (EXACT) (12)
+ ~ ANYOF[Rr] (END) (14)
+ ~ attach to TAIL (25) offset to 11
+ >(foo|bar)$< tail~ EXACT <x> (1)
+ piec
+ atom
+ >foo|bar)$< reg
+ 28 brnc
piec
atom
- >< 37 tail~ OPEN1 (26)
- ~ BRANCH (28)
- ~ BRANCH (31)
- ~ CLOSE1 (34)
- 38 tsdy~ EXACT <x> (EXACT) (1)
- ~ BRANCH (END) (3)
- ~ BRANCH (END) (9)
- ~ TAIL (END) (25)
- ~ OPEN1 (END) (26)
- ~ BRANCH (END) (28)
- ~ BRANCH (END) (31)
- ~ CLOSE1 (END) (34)
- ~ EOL (END) (36)
- ~ attach to END (37) offset to 1<div></div>
+ >|bar)$< 31 tail~ OPEN1 (26)
+ >bar)$< brnc
+ 32 piec
+ atom
+ >)$< 34 tail~ BRANCH (28)
+ 36 tsdy~ BRANCH (END) (31)
+ ~ attach to CLOSE1 (34) offset to 3
+ tsdy~ EXACT <foo> (EXACT) (29)
+ ~ attach to CLOSE1 (34) offset to 5
+ tsdy~ EXACT <bar> (EXACT) (32)
+ ~ attach to CLOSE1 (34) offset to 2
+ >$< tail~ BRANCH (3)
+ ~ BRANCH (9)
+ ~ TAIL (25)
+ piec
+ atom
+ >< 37 tail~ OPEN1 (26)
+ ~ BRANCH (28)
+ ~ BRANCH (31)
+ ~ CLOSE1 (34)
+ 38 tsdy~ EXACT <x> (EXACT) (1)
+ ~ BRANCH (END) (3)
+ ~ BRANCH (END) (9)
+ ~ TAIL (END) (25)
+ ~ OPEN1 (END) (26)
+ ~ BRANCH (END) (28)
+ ~ BRANCH (END) (31)
+ ~ CLOSE1 (END) (34)
+ ~ EOL (END) (36)
+ ~ attach to END (37) offset to 1
Resulting in the program
37: END(0)
Here we can see a much more complex program, with various optimisations in
-play. At regnode 10 we can see an example where a char class with only
-one character in it was turned into an EXACT node. We can also see where
-an entire alternation was turned into a TRIE-EXACT node. As a consequence
+play. At regnode 10 we see an example where a character class with only
+one character in it was turned into an C<EXACT> node. We can also see where
+an entire alternation was turned into a C<TRIE-EXACT> node. As a consequence,
some of the regnodes have been marked as optimised away. We can see that
-the C<$> symbol has been converted into an EOL regop, a special piece of
-code that looks for \n or the end of a string.
+the C<$> symbol has been converted into an C<EOL> regop, a special piece of
+code that looks for C<\n> or the end of the string.
-The next pointer for BRANCHes is interesting in that it points at where
-execution should go if the branch fails. When executing if the engine
-tries to traverse from a branch to a regnext that isnt a branch then
-the engine will know the overall series of branches have failed.
+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
+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 has failed.
=head3 Peep-hole Optimisation and Analysis
The C<(a|b)*> part can match at every char in the string, and then fail
every time because there is no C<z> in the string. So obviously we can
-not bother to use the regex engine unless there is a 'z' in the string.
+avoid using the regex engine unless there is a C<z> in the string.
Likewise in a pattern like:
/foo(\w+)bar/
In this case we know that the string must contain a C<foo> which must be
-followed by C<bar>. We can use Fast Boyer-More matching as implemented
-in fbm_instr() to find the location of these strings. If they dont exist
-then we dont need to resort to the much more expensive regex engine.
-Even better if they do exist then we can use their positions to
+followed by C<bar>. We can use Fast Boyer-Moore matching as implemented
+in C<fbm_instr()> to find the location of these strings. If they don't exist
+then we don't need to resort to the much more expensive regex engine.
+Even better, if they do exist then we can use their positions to
reduce the search space that the regex engine needs to cover to determine
-if the entire pattern does match.
+if the entire pattern matches.
There are various aspects of the pattern that can be used to facilitate
optimisations along these lines:
- * anchored fixed strings
- * floating fixed strings
- * minimum and maximum length requirements
- * start class
- * Beginning/End of line positions
+=over 5
+
+=item * anchored fixed strings
-Another form of optimisation that can occur is post-parse "peep-hole"
-optimisations, where inefficient constructs are modified so that they
-are more efficient. An example of this is 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
-TAIL point to thing the TAIL points to, in essence "skipping" the node.
+=item * floating fixed strings
-Another optimisation that can occur is that of "EXACT merging" which is
-where two consecutive EXACT nodes are merged into a single more efficient
-to execute regop. An even more agressive form of this is that a branch
-sequence of the form EXACT BRANCH ... EXACT can be converted into a TRIE
-regop.
+=item * minimum and maximum length requirements
-All of this occurs in the routine study_chunk() which uses a special
-structure scan_data_t to store the analysis that it has performed, and
-as it goes does the "peep-hole" optimisations.
+=item * start class
-The code involved in study_chunk() is extremely cryptic. Be careful. :-)
+=item * Beginning/End of line positions
+
+=back
+
+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
+regop. An even more aggressive form of this is that a branch
+sequence of the form C<EXACT BRANCH ... EXACT> can be converted into a
+C<TRIE-EXACT> regop.
+
+All of this occurs in the routine C<study_chunk()> which uses a special
+structure C<scan_data_t> to store the analysis that it has performed, and
+does the "peep-hole" optimisations as it goes.
+
+The code involved in C<study_chunk()> is extremely cryptic. Be careful. :-)
=head2 Execution
finding the start point in the string where we should match from,
and the second being running the regop interpreter.
-If we can tell that there is no valid start point we don't bother running
-interpreter at all. Likewise if we know from the analysis phase that we
-can not optimise detection of the start position we go straight to the
+If we can tell that there is no valid start point then we don't bother running
+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 re_intuit_start() and pregexec(). These routines
+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 pregexec() may even call re_intuit_start() on its own. Nevertheless
-the perl source code may call into either, or both.
+and C<pregexec()> may even call C<re_intuit_start()> on its own. Nevertheless
+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 Mitchel in blead perl it no longer is. Instead 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 which means 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.
=head3 Start position and no-match optimisations
-re_intuit_start() is responsible for handling start point and no match
+C<re_intuit_start()> is responsible for handling start points and no-match
optimisations as determined by the results of the analysis done by
-
study_chunk() (and described in L<Peep-hole Optimisation and Analysis>).
+
C<study_chunk()> (and described in L<Peep-hole Optimisation and Analysis>).
-The basic structure of this routine is to try to find the start and/or
-end points of where the pattern could match, and to ensure that the string
-is long enough to match the pattern. It tries to use more efficent
-methods over less efficient methods and may involve considerable cross
-checking of constraints to find the place in the string that matches.
+The basic structure of this routine is to try to find the start- and/or
+end-points of where the pattern could match, and to ensure that the string
+is long enough to match the pattern. It tries to use more efficient
+methods over less efficient methods and may involve considerable
+cross-checking of constraints to find the place in the string that matches.
For instance it may try to determine that a given fixed string must be
not only present but a certain number of chars before the end of the
string, or whatever.
-It calls out into several other routines, like fbm_instr() which does
-"Fast Boyer More" matching and find_byclass() which is responsible for
+It calls several other routines, such as C<fbm_instr()> which does
+Fast Boyer Moore matching and C<find_byclass()> which is responsible for
finding the start using the first mandatory regop in the program.
-When the optimisation criteria have been satisfied reg_try() is called
+When the optimisation criteria have been satisfied, C<reg_try()> is called
to perform the match.
=head3 Program execution
C<pregexec()> is the main entry point for running a regex. It contains
-support for initializing the regex interpreters state, running
-re_intuit_start() if needed, and running the intepreter on the string
-from various start positions as needed. When its necessary to use
+support for initialising the regex interpreter's state, running
+C<re_intuit_start()> if needed, and running the interpreter on the string
+from various start positions as needed. When it is necessary to use
the regex interpreter C<pregexec()> calls C<regtry()>.
C<regtry()> is the entry point into the regex interpreter. It expects
-as arguments a pointer to a regmatch_info structure and a pointer to
+as arguments a pointer to a C<regmatch_info> structure and a pointer to
a string. It returns an integer 1 for success and a 0 for failure.
-It is basically a setup wrapper around C<regmatch()>.
+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 Localization Support
+=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
+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
+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.)
-No matter how you look at it unicode support is going to be a pain in a
+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
-characters often won't scale to handle the size of the 'utf8' character
+characters often won't scale to handle the size of the UTF-8 character
set. Things you can take for granted with ASCII may not be true with
-unicode. For instance in ASCII its safe to assume that
-C<sizeof(char1) == sizeof(char2)>, in utf8 it isn't. Unicode case folding is
-vastly more complex than the simple rules of English, and even when not
-using unicode but only localized single byte encodings things can get
-tricky (technically GERMAN-SHARP-ESS should match 'ss' in localized case
-insensitive matching.)
-
-Making things worse is that C<utf8> support was a later addition to the
-regex engine (as it was to perl) and necessarily this made things a lot
-more complicated. Obviously its easier to design a regex engine with
-unicode support from the beginning than it is to retrofit one that
-wasn't designed with it in mind.
-
-Pretty well every regop that involves looking at the input string has
-two cases, one for 'utf8' and one not. In fact its often more complex
-than that, as the pattern may be 'utf8' as well.
+Unicode. For instance, in ASCII, it is safe to assume that
+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, 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
+more complicated. Obviously it is easier to design a regex engine with
+Unicode support in mind from the beginning than it is to retrofit it to
+one that wasn't.
+
+Nearly all regops that involve looking at the input string have
+two cases, one for UTF-8, and one not. In fact, it's often more complex
+than that, as the pattern may be UTF-8 as well.
Care must be taken when making changes to make sure that you handle
-utf8 properly both at compile time and at execution time, including
+UTF-8 properly, both at compile time and at execution time, including
when the string and pattern are mismatched.
-The following comment in regcomp.h gives an example of exactly how
-tricky this can be:
+=head2 Base Structures
+
+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>.
+
+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 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
+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.
- Two problematic code points in Unicode casefolding of EXACT nodes:
+=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<offsets>
+
+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 C<regstclass>
+
+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 C<data>
+
+This field points at a C<reg_data> structure, which is defined as follows
+
+ struct reg_data {
+ U32 count;
+ U8 *what;
+ void* data[1];
+ };
- U+0390 - GREEK SMALL LETTER IOTA WITH DIALYTIKA AND TONOS
- U+03B0 - GREEK SMALL LETTER UPSILON WITH DIALYTIKA AND TONOS
+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.
- which casefold to
+=item C<program>
- Unicode UTF-8
+Compiled program. Inlined into the structure so the entire struct can be
+treated as a single blob.
- U+03B9 U+0308 U+0301 0xCE 0xB9 0xCC 0x88 0xCC 0x81
- U+03C5 U+0308 U+0301 0xCF 0x85 0xCC 0x88 0xCC 0x81
+=back
- 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.
+=head1 SEE ALSO
- 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).
+L<perlreapi>
- 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.
+L<perlre>
+
+L<perlunitut>
=head1 AUTHOR
With excerpts from Perl, and contributions and suggestions from
Ronald J. Kimball, Dave Mitchell, Dominic Dunlop, Mark Jason Dominus,
-and Stephen McCamant.
+Stephen McCamant, and David Landgren.
-=head1 LICENSE
+=head1 LICENCE
Same terms as Perl.
=head1 REFERENCES
-[1] http://perl.plover.com/Rx/paper/
+[1] L<http://perl.plover.com/Rx/paper/>
+
+[2] L<http://www.unicode.org>
=cut
https://perl5.git.perl.org / perl5.git / blobdiff