pcre2jit(3) — Linux manual page


PCRE2JIT(3)             Library Functions Manual             PCRE2JIT(3)

NAME         top

       PCRE2 - Perl-compatible regular expressions (revised API)


       Just-in-time compiling is a heavyweight optimization that can
       greatly speed up pattern matching. However, it comes at the cost
       of extra processing 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 necessarily mean many calls of a matching
       function; if the pattern is not anchored, matching attempts may
       take place many times at various positions 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

       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 JIT support was written by
       Zoltan Herczeg.


       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 hardware platforms:

         ARM 32-bit (v5, v7, and Thumb2)
         ARM 64-bit
         IBM s390x 64 bit
         Intel x86 32-bit and 64-bit
         LoongArch 64 bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         RISC-V 32-bit and 64-bit

       If --enable-jit is set on an unsupported platform, compilation

       A client program can tell if JIT support is available by calling
       pcre2_config() with the PCRE2_CONFIG_JIT option. The result is
       one if PCRE2 was built with JIT support, and zero otherwise.
       However, having the JIT code available does not guarantee that it
       will be used for any particular match. One reason for this is
       that there are a number of options and pattern items that are not
       supported by JIT (see below). Another reason is that in some
       environments JIT is unable to get memory in which to build its
       compiled code. The only guarantee from pcre2_config() is that if
       it returns zero, JIT will definitely not be used.

       A simple program does not need to check availability in order to
       use JIT when possible. The API is implemented in a way that falls
       back to the interpretive code if JIT is not available or cannot
       be used for a given match. For programs that need the best
       possible performance, there is a "fast path" API that is JIT-

SIMPLE USE OF JIT         top

       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_COMPLETE, PCRE2_JIT_PARTIAL_HARD, or

       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 documented 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_JIT_COMPLETE requests the JIT compiler to generate code for
       complete matches. If you want to run partial matches using the
       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 available.
       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_COMPLETE 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 matching. If
       pcre2_jit_compile() is called with no option bits set, it
       immediately returns zero. This is an alternative way of testing
       whether JIT is available.

       At present, it is not possible to free JIT compiled code except
       when the entire compiled pattern is freed by calling

       In some circumstances you may need to call additional functions.
       These are described in the section entitled "Controlling the JIT
       stack" below.

       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
       generated. You can find out if JIT compilation was successful for
       a compiled 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. Successful JIT compilation does not, however,
       guarantee the use of JIT at match time because there are some
       match time options that are not supported by JIT.


       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 generated 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. The calling program may crash or
       loop or otherwise misbehave.

       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
       interpreter in pcre2_match() to support invalid UTF, and, if
       pcre2_jit_compile() is subsequently 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
       compatibility.  It is superseded by the pcre2_compile() option
       PCRE2_MATCH_INVALID_UTF and should no longer be used. It may be
       removed in future.


       The pcre2_match() options that are supported for JIT matching are
       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 assertion condition in a conditional group.


       When a pattern is matched using JIT, 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 "Controlling 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.


       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"

       The pcre2_jit_stack_create() function creates a JIT stack. Its
       arguments 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 technically minded:
       the address space is allocated by mmap or VirtualAlloc.) A
       maximum stack size of 512KiB to 1MiB should be more than enough
       for any pattern.

       The pcre2_jit_stack_assign() function specifies which stack JIT
       code should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is
       subsequently passed to a matching function, its information
       determines which JIT stack is used. If this argument is NULL, the
       function returns immediately, 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
             on the machine stack is used. This is the default when a
             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
             called with data as an argument at the start of matching,
             order to set up a JIT stack. If the return from the
             function is NULL, the internal 32KiB stack is used;
       otherwise the
             return value must be a valid JIT stack, the result of

       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 incompatible for JIT matching. A callback function can
       therefore be used to determine whether a match operation was
       executed by JIT or by the interpreter.

       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 callout 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 recommended.

       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
           return thread_local_var

       All the functions described in this section do nothing if JIT is
       not available.

JIT STACK FAQ         top

       (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 difficult. 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 memory 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 if needed.

       (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, anytime.  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 pcre2_match()?

       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
       memory 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 memory (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.


       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is
       possible. 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
       context, for custom memory management, or NULL for standard
       memory management.

EXAMPLE CODE         top

       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.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         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,
         /* Process result */


JIT FAST PATH API         top

       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 and PCRE2_ENDANCHORED) 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
       (partial 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 example, 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. If invalid UTF data is passed when
       PCRE2_MATCH_INVALID_UTF was not set for pcre2_compile(), the
       result is undefined. The program may crash or loop or give wrong
       results. In the absence of PCRE2_MATCH_INVALID_UTF you should
       call pcre2_jit_match() in UTF mode only if you are sure the
       subject is valid.

       Bypassing the sanity checks and the pcre2_match() wrapping can
       give speedups of more than 10%.

SEE ALSO         top

       pcre2api(3), pcre2unicode(3)

AUTHOR         top

       Philip Hazel (FAQ by Zoltan Herczeg)
       Retired from University Computing Service
       Cambridge, England.

REVISION         top

       Last updated: 23 January 2023
       Copyright (c) 1997-2023 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular
       Expressions) project.  Information about the project can be found
       at ⟨http://www.pcre.org/⟩.  If you have a bug report for this
       manual page, see
       ⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩.  This page was
       obtained from the tarball fetched from
       ⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2023-12-22.  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to man-pages@man7.org

PCRE2 10.43                  23 January 2023                 PCRE2JIT(3)