PCRE2JIT(3) Library Functions Manual PCRE2JIT(3)
PCRE2 - Perl-compatible regular expressions (revised API)
PCRE2 JUST-IN-TIME COMPILER SUPPORT
Just-in-time compiling is a heavyweight optimization that can greatly
speed up pattern matching. However, it comes at the cost of extra pro-
cessing before the match is performed, so it is of most benefit when
the same pattern is going to be matched many times. This does not nec-
essarily mean many calls of a matching function; if the pattern is not
anchored, matching attempts may take place many times at various posi-
tions in the subject, even for a single call. Therefore, if the subject
string is very long, it may still pay to use JIT even for one-off
matches. JIT support is available for all of the 8-bit, 16-bit and
32-bit PCRE2 libraries.
JIT support applies only to the traditional Perl-compatible matching
function. It does not apply when the DFA matching function is being
used. The code for this support was written by Zoltan Herczeg.
AVAILABILITY OF JIT SUPPORT
JIT support is an optional feature of PCRE2. The "configure" option
--enable-jit (or equivalent CMake option) must be set when PCRE2 is
built if you want to use JIT. The support is limited to the following
ARM 32-bit (v5, v7, and Thumb2)
IBM s390x 64 bit
Intel x86 32-bit and 64-bit
MIPS 32-bit and 64-bit
Power PC 32-bit and 64-bit
If --enable-jit is set on an unsupported platform, compilation fails.
A program can tell if JIT support is available by calling pcre2_con-
fig() with the PCRE2_CONFIG_JIT option. The result is 1 when JIT is
available, and 0 otherwise. However, a simple program does not need to
check this in order to use JIT. The API is implemented in a way that
falls back to the interpretive code if JIT is not available. For pro-
grams that need the best possible performance, there is also a "fast
path" API that is JIT-specific.
SIMPLE USE OF JIT
To make use of the JIT support in the simplest way, all you have to do
is to call pcre2_jit_compile() after successfully compiling a pattern
with pcre2_compile(). This function has two arguments: the first is the
compiled pattern pointer that was returned by pcre2_compile(), and the
second is zero or more of the following option bits: PCRE2_JIT_COM-
PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.
If JIT support is not available, a call to pcre2_jit_compile() does
nothing and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
pattern is passed to the JIT compiler, which turns it into machine code
that executes much faster than the normal interpretive code, but yields
exactly the same results. The returned value from pcre2_jit_compile()
is zero on success, or a negative error code.
There is a limit to the size of pattern that JIT supports, imposed by
the size of machine stack that it uses. The exact rules are not docu-
mented because they may change at any time, in particular, when new
optimizations are introduced. If a pattern is too big, a call to
pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.
PCRE2_JIT_COMPLETE requests the JIT compiler to generate code for com-
plete matches. If you want to run partial matches using the PCRE2_PAR-
TIAL_HARD or PCRE2_PARTIAL_SOFT options of pcre2_match(), you should
set one or both of the other options as well as, or instead of
PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
for each of the three modes (normal, soft partial, hard partial). When
pcre2_match() is called, the appropriate code is run if it is avail-
able. Otherwise, the pattern is matched using interpretive code.
You can call pcre2_jit_compile() multiple times for the same compiled
pattern. It does nothing if it has previously compiled code for any of
the option bits. For example, you can call it once with PCRE2_JIT_COM-
PLETE and (perhaps later, when you find you need partial matching)
again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time it
will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
ing. If pcre2_jit_compile() is called with no option bits set, it imme-
diately returns zero. This is an alternative way of testing whether JIT
At present, it is not possible to free JIT compiled code except when
the entire compiled pattern is freed by calling pcre2_code_free().
In some circumstances you may need to call additional functions. These
are described in the section entitled "Controlling the JIT stack"
There are some pcre2_match() options that are not supported by JIT, and
there are also some pattern items that JIT cannot handle. Details are
given below. In both cases, matching automatically falls back to the
interpretive code. If you want to know whether JIT was actually used
for a particular match, you should arrange for a JIT callback function
to be set up as described in the section entitled "Controlling the JIT
stack" below, even if you do not need to supply a non-default JIT
stack. Such a callback function is called whenever JIT code is about to
be obeyed. If the match-time options are not right for JIT execution,
the callback function is not obeyed.
If the JIT compiler finds an unsupported item, no JIT data is gener-
ated. You can find out if JIT matching is available after compiling a
pattern by calling pcre2_pattern_info() with the PCRE2_INFO_JITSIZE
option. A non-zero result means that JIT compilation was successful. A
result of 0 means that JIT support is not available, or the pattern was
not processed by pcre2_jit_compile(), or the JIT compiler was not able
to handle the pattern.
MATCHING SUBJECTS CONTAINING INVALID UTF
When a pattern is compiled with the PCRE2_UTF option, subject strings
are normally expected to be a valid sequence of UTF code units. By
default, this is checked at the start of matching and an error is gen-
erated if invalid UTF is detected. The PCRE2_NO_UTF_CHECK option can be
passed to pcre2_match() to skip the check (for improved performance) if
you are sure that a subject string is valid. If this option is used
with an invalid string, the result is undefined.
However, a way of running matches on strings that may contain invalid
UTF sequences is available. Calling pcre2_compile() with the
PCRE2_MATCH_INVALID_UTF option has two effects: it tells the inter-
preter in pcre2_match() to support invalid UTF, and, if pcre2_jit_com-
pile() is called, the compiled JIT code also supports invalid UTF.
Details of how this support works, in both the JIT and the interpretive
cases, is given in the pcre2unicode documentation.
There is also an obsolete option for pcre2_jit_compile() called
PCRE2_JIT_INVALID_UTF, which currently exists only for backward compat-
ibility. It is superseded by the pcre2_compile() option
PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be removed
UNSUPPORTED OPTIONS AND PATTERN ITEMS
The pcre2_match() options that are supported for JIT matching are
PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and
PCRE2_PARTIAL_SOFT. The PCRE2_ANCHORED and PCRE2_ENDANCHORED options
are not supported at match time.
If the PCRE2_NO_JIT option is passed to pcre2_match() it disables the
use of JIT, forcing matching by the interpreter code.
The only unsupported pattern items are \C (match a single data unit)
when running in a UTF mode, and a callout immediately before an asser-
tion condition in a conditional group.
RETURN VALUES FROM JIT MATCHING
When a pattern is matched using JIT matching, the return values are the
same as those given by the interpretive pcre2_match() code, with the
addition of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means
that the memory used for the JIT stack was insufficient. See "Control-
ling the JIT stack" below for a discussion of JIT stack usage.
The error code PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if
searching a very large pattern tree goes on for too long, as it is in
the same circumstance when JIT is not used, but the details of exactly
what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code
is never returned when JIT matching is used.
CONTROLLING THE JIT STACK
When the compiled JIT code runs, it needs a block of memory to use as a
stack. By default, it uses 32KiB on the machine stack. However, some
large or complicated patterns need more than this. The error
PCRE2_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
Three functions are provided for managing blocks of memory for use as
JIT stacks. There is further discussion about the use of JIT stacks in
the section entitled "JIT stack FAQ" below.
The pcre2_jit_stack_create() function creates a JIT stack. Its argu-
ments are a starting size, a maximum size, and a general context (for
memory allocation functions, or NULL for standard memory allocation).
It returns a pointer to an opaque structure of type pcre2_jit_stack, or
NULL if there is an error. The pcre2_jit_stack_free() function is used
to free a stack that is no longer needed. If its argument is NULL, this
function returns immediately, without doing anything. (For the techni-
cally minded: the address space is allocated by mmap or VirtualAlloc.)
A maximum stack size of 512KiB to 1MiB should be more than enough for
The pcre2_jit_stack_assign() function specifies which stack JIT code
should use. Its arguments are as follows:
The first argument is a pointer to a match context. When this is subse-
quently passed to a matching function, its information determines which
JIT stack is used. If this argument is NULL, the function returns imme-
diately, without doing anything. There are three cases for the values
of the other two options:
(1) If callback is NULL and data is NULL, an internal 32KiB block
on the machine stack is used. This is the default when a match
context is created.
(2) If callback is NULL and data is not NULL, data must be
a pointer to a valid JIT stack, the result of calling
(3) If callback is not NULL, it must point to a function that is
called with data as an argument at the start of matching, in
order to set up a JIT stack. If the return from the callback
function is NULL, the internal 32KiB stack is used; otherwise the
return value must be a valid JIT stack, the result of calling
A callback function is obeyed whenever JIT code is about to be run; it
is not obeyed when pcre2_match() is called with options that are incom-
patible for JIT matching. A callback function can therefore be used to
determine whether a match operation was executed by JIT or by the
You may safely use the same JIT stack for more than one pattern (either
by assigning directly or by callback), as long as the patterns are
matched sequentially in the same thread. Currently, the only way to set
up non-sequential matches in one thread is to use callouts: if a call-
out function starts another match, that match must use a different JIT
stack to the one used for currently suspended match(es).
In a multithread application, if you do not specify a JIT stack, or if
you assign or pass back NULL from a callback, that is thread-safe,
because each thread has its own machine stack. However, if you assign
or pass back a non-NULL JIT stack, this must be a different stack for
each thread so that the application is thread-safe.
Strictly speaking, even more is allowed. You can assign the same non-
NULL stack to a match context that is used by any number of patterns,
as long as they are not used for matching by multiple threads at the
same time. For example, you could use the same stack in all compiled
patterns, with a global mutex in the callback to wait until the stack
is available for use. However, this is an inefficient solution, and not
This is a suggestion for how a multithreaded program that needs to set
up non-default JIT stacks might operate:
During thread initialization
thread_local_var = pcre2_jit_stack_create(...)
During thread exit
Use a one-line callback function
All the functions described in this section do nothing if JIT is not
JIT STACK FAQ
(1) Why do we need JIT stacks?
PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
where the local data of the current node is pushed before checking its
child nodes. Allocating real machine stack on some platforms is diffi-
cult. For example, the stack chain needs to be updated every time if we
extend the stack on PowerPC. Although it is possible, its updating
time overhead decreases performance. So we do the recursion in memory.
(2) Why don't we simply allocate blocks of memory with malloc()?
Modern operating systems have a nice feature: they can reserve an
address space instead of allocating memory. We can safely allocate mem-
ory pages inside this address space, so the stack could grow without
moving memory data (this is important because of pointers). Thus we can
allocate 1MiB address space, and use only a single memory page (usually
4KiB) if that is enough. However, we can still grow up to 1MiB anytime
(3) Who "owns" a JIT stack?
The owner of the stack is the user program, not the JIT studied pattern
or anything else. The user program must ensure that if a stack is being
used by pcre2_match(), (that is, it is assigned to a match context that
is passed to the pattern currently running), that stack must not be
used by any other threads (to avoid overwriting the same memory area).
The best practice for multithreaded programs is to allocate a stack for
each thread, and return this stack through the JIT callback function.
(4) When should a JIT stack be freed?
You can free a JIT stack at any time, as long as it will not be used by
pcre2_match() again. When you assign the stack to a match context, only
a pointer is set. There is no reference counting or any other magic.
You can free compiled patterns, contexts, and stacks in any order, any-
time. Just do not call pcre2_match() with a match context pointing to
an already freed stack, as that will cause SEGFAULT. (Also, do not free
a stack currently used by pcre2_match() in another thread). You can
also replace the stack in a context at any time when it is not in use.
You should free the previous stack before assigning a replacement.
(5) Should I allocate/free a stack every time before/after calling
No, because this is too costly in terms of resources. However, you
could implement some clever idea which release the stack if it is not
used in let's say two minutes. The JIT callback can help to achieve
this without keeping a list of patterns.
(6) OK, the stack is for long term memory allocation. But what happens
if a pattern causes stack overflow with a stack of 1MiB? Is that 1MiB
kept until the stack is freed?
Especially on embedded sytems, it might be a good idea to release mem-
ory sometimes without freeing the stack. There is no API for this at
the moment. Probably a function call which returns with the currently
allocated memory for any stack and another which allows releasing mem-
ory (shrinking the stack) would be a good idea if someone needs this.
(7) This is too much of a headache. Isn't there any better solution for
JIT stack handling?
No, thanks to Windows. If POSIX threads were used everywhere, we could
throw out this complicated API.
FREEING JIT SPECULATIVE MEMORY
void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);
The JIT executable allocator does not free all memory when it is possi-
ble. It expects new allocations, and keeps some free memory around to
improve allocation speed. However, in low memory conditions, it might
be better to free all possible memory. You can cause this to happen by
calling pcre2_jit_free_unused_memory(). Its argument is a general con-
text, for custom memory management, or NULL for standard memory manage-
This is a single-threaded example that specifies a JIT stack without
using a callback. A real program should include error checking after
all the function calls.
re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
&errornumber, &erroffset, NULL);
rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
mcontext = pcre2_match_context_create(NULL);
jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
match_data = pcre2_match_data_create(re, 10);
rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
/* Process result */
JIT FAST PATH API
Because the API described above falls back to interpreted matching when
JIT is not available, it is convenient for programs that are written
for general use in many environments. However, calling JIT via
pcre2_match() does have a performance impact. Programs that are written
for use where JIT is known to be available, and which need the best
possible performance, can instead use a "fast path" API to call JIT
matching directly instead of calling pcre2_match() (obviously only for
patterns that have been successfully processed by pcre2_jit_compile()).
The fast path function is called pcre2_jit_match(), and it takes
exactly the same arguments as pcre2_match(). However, the subject
string must be specified with a length; PCRE2_ZERO_TERMINATED is not
supported. Unsupported option bits (for example, PCRE2_ANCHORED,
PCRE2_ENDANCHORED and PCRE2_COPY_MATCHED_SUBJECT) are ignored, as is
the PCRE2_NO_JIT option. The return values are also the same as for
pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (par-
tial or complete) is requested that was not compiled.
When you call pcre2_match(), as well as testing for invalid options, a
number of other sanity checks are performed on the arguments. For exam-
ple, if the subject pointer is NULL but the length is non-zero, an
immediate error is given. Also, unless PCRE2_NO_UTF_CHECK is set, a UTF
subject string is tested for validity. In the interests of speed, these
checks do not happen on the JIT fast path, and if invalid data is
passed, the result is undefined.
Bypassing the sanity checks and the pcre2_match() wrapping can give
speedups of more than 10%.
Philip Hazel (FAQ by Zoltan Herczeg)
University Computing Service
Last updated: 30 November 2021
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PCRE2 10.40 30 November 2021 PCRE2JIT(3)
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