Description:
------------
I don't have a small reproducer to share yet (working on this), but at least I was able to catch a backtrace:

# gdb /usr/lib64/php7.3/bin/php-fpm
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Reading symbols from /usr/lib64/php7.3/bin/php-fpm...
Reading symbols from /usr/lib64/debug//usr/lib64/php7.3/bin/php-fpm.debug...
(gdb)
(gdb) set follow-fork-mode child
(gdb) r --nodaemonize --fpm-config /etc/php/fpm-php7.3/php-fpm.debug.conf
Starting program: /usr/lib64/php7.3/bin/php-fpm --nodaemonize --fpm-config /etc/php/fpm-php7.3/php-fpm.debug.conf
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[05-Jul-2019 17:51:27] NOTICE: fpm is running, pid 23812
[Attaching after Thread 0x7ffff480a980 (LWP 23812) fork to child process 23819]
[New inferior 2 (process 23819)]
[Detaching after fork from parent process 23812]
[Inferior 1 (process 23812) detached]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
[05-Jul-2019 17:51:27] NOTICE: ready to handle connections

Thread 2.1 "php-fpm" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff480a980 (LWP 23819)]
zend_mm_alloc_small (bin_num=6, size=56, heap=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_alloc.c:1289
1289    /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_alloc.c: No such file or directory.
(gdb) bt
#0  zend_mm_alloc_small (bin_num=6, size=56, heap=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_alloc.c:1289
#1  _emalloc_56 () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_alloc.c:2423
#2  0x0000555555a01dc9 in _zend_new_array_0 () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_hash.c:217
#3  0x000055555592d361 in zif_explode (execute_data=0x7ffff46228c0, return_value=0x7ffff46228a0)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/standard/string.c:1173
#4  0x0000555555a7912f in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:690
#5  execute_ex (ex=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:55465
#6  0x00005555559e8bc1 in zend_call_function (fci=fci@entry=0x7fffffffac70, fci_cache=0x7fffb37d2a68, fci_cache@entry=0x7fffffffac50)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_execute_API.c:756
#7  0x000055555571ae95 in preg_do_repl_func (mark=<optimized out>, count=<optimized out>, subpat_names=0x0, offsets=0x55555662f390,
    subject=0x7fff927cf418 "[apress_testimonial by=\"Max Mustermann\" designation=\"t5 Content\" testimonialbordercolor=\"#eaeaea\" testimonialauthorcolor=\"#333333\"]Digitaler Content benötigt authentische Menschen, die diesen verfas"..., fcc=0x7fffffffac50, fci=0x7fffffffac70)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1500
#8  php_pcre_replace_func_impl (pce=0x555556631ed0, subject_str=0x7fff927cf400,
    subject=0x7fff927cf418 "[apress_testimonial by=\"Max Mustermann\" designation=\"t5 Content\" testimonialbordercolor=\"#eaeaea\" testimonialauthorcolor=\"#333333\"]Digitaler Content benötigt authentische Menschen, die diesen verfas"..., subject_len=1592, fci=0x7fffffffac70, fcc=0x7fffffffac50,
    limit=18446744073709551614, replace_count=0x7fffffffabb8) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1891
#9  0x000055555571c20c in php_pcre_replace_func (replace_count=0x7fffffffabb8, limit=18446744073709551615, fcc=0x7fffffffac50, fci=0x7fffffffac70,
    subject_str=0x7fff927cf400, regex=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2018
#10 php_replace_in_subject_func (fci=fci@entry=0x7fffffffac70, fcc=fcc@entry=0x7fffffffac50, subject=subject@entry=0x7ffff4621d60,
    limit=limit@entry=18446744073709551615, replace_count=0x7fffffffabb8, regex=<optimized out>, regex=<optimized out>)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2145
#11 0x000055555571c487 in preg_replace_func_impl (regex=regex@entry=0x7ffff4621d40, fci=fci@entry=0x7fffffffac70, fcc=fcc@entry=0x7fffffffac50,
    subject=subject@entry=0x7ffff4621d60, limit_val=-1, return_value=<optimized out>, return_value=<optimized out>)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2195
#12 0x000055555571c91e in zif_preg_replace_callback (execute_data=0x7ffff4621cf0, return_value=0x7ffff4621cc0)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2358
#13 0x0000555555a7912f in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:690
#14 execute_ex (ex=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:55465
#15 0x00005555559e8bc1 in zend_call_function (fci=fci@entry=0x7fffffffb0e0, fci_cache=0x7fffb3d80b90, fci_cache@entry=0x7fffffffb0c0)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_execute_API.c:756
#16 0x000055555571ae95 in preg_do_repl_func (mark=<optimized out>, count=<optimized out>, subpat_names=0x0, offsets=0x55555662ec50,
    subject=0x7fff92306018 "[vc_column width=\"1/3\" offset=\"vc_hidden-sm vc_hidden-xs\"][apress_testimonial by=\"Max Mustermann\" designation=\"t5 Content\" testimonialbordercolor=\"#eaeaea\" testimonialauthorcolor=\"#333333\"]Digitaler "..., fcc=0x7fffffffb0c0, fci=0x7fffffffb0e0)
    at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1500
#17 php_pcre_replace_func_impl (pce=0x555556631c40, subject_str=0x7fff92306000,
    subject=0x7fff92306018 "[vc_column width=\"1/3\" offset=\"vc_hidden-sm vc_hidden-xs\"][apress_testimonial by=\"Max Mustermann\" designation=\"t5 Content\" testimonialbordercolor=\"#eaeaea\" testimonialauthorcolor=\"#333333\"]Digitaler "..., subject_len=5021, fci=0x7fffffffb0e0, fcc=0x7fffffffb0c0,
--Type <RET> for more, q to quit, c to continue without paging--c
    limit=18446744073709551615, replace_count=0x7fffffffb028) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1891
#18 0x000055555571c20c in php_pcre_replace_func (replace_count=0x7fffffffb028, limit=18446744073709551615, fcc=0x7fffffffb0c0, fci=0x7fffffffb0e0, subject_str=0x7fff92306000, regex=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2018
#19 php_replace_in_subject_func (fci=fci@entry=0x7fffffffb0e0, fcc=fcc@entry=0x7fffffffb0c0, subject=subject@entry=0x7ffff46212c0, limit=limit@entry=18446744073709551615, replace_count=0x7fffffffb028, regex=<optimized out>, regex=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2145
#20 0x000055555571c487 in preg_replace_func_impl (regex=regex@entry=0x7ffff46212a0, fci=fci@entry=0x7fffffffb0e0, fcc=fcc@entry=0x7fffffffb0c0, subject=subject@entry=0x7ffff46212c0, limit_val=-1, return_value=<optimized out>, return_value=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2195
#21 0x000055555571c91e in zif_preg_replace_callback (execute_data=0x7ffff4621250, return_value=0x7ffff4621220) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2358
#22 0x0000555555a7912f in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:690
#23 execute_ex (ex=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:55465
#24 0x00005555559e8bc1 in zend_call_function (fci=fci@entry=0x7fffffffb550, fci_cache=0x7fffb3d80b90, fci_cache@entry=0x7fffffffb530) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_execute_API.c:756
#25 0x000055555571ae95 in preg_do_repl_func (mark=<optimized out>, count=<optimized out>, subpat_names=0x0, offsets=0x55555642b5a0, subject=0x7fff922f4018 "[vc_row full_width=\"stretch_row\" full_height=\"yes\" content_placement=\"middle\" apress_enable_separator=\"yes\" apress_separator_height=\"300\" apress_enable_row_overlay=\"\" css=\".vc_custom_1542919100842{mar"..., fcc=0x7fffffffb530, fci=0x7fffffffb550) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1500
#26 php_pcre_replace_func_impl (pce=0x55555662da90, subject_str=0x7fff922f4000, subject=0x7fff922f4018 "[vc_row full_width=\"stretch_row\" full_height=\"yes\" content_placement=\"middle\" apress_enable_separator=\"yes\" apress_separator_height=\"300\" apress_enable_row_overlay=\"\" css=\".vc_custom_1542919100842{mar"..., subject_len=28732, fci=0x7fffffffb550, fcc=0x7fffffffb530, limit=18446744073709551604, replace_count=0x7fffffffb498) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:1891
#27 0x000055555571c20c in php_pcre_replace_func (replace_count=0x7fffffffb498, limit=18446744073709551615, fcc=0x7fffffffb530, fci=0x7fffffffb550, subject_str=0x7fff922f4000, regex=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2018
#28 php_replace_in_subject_func (fci=fci@entry=0x7fffffffb550, fcc=fcc@entry=0x7fffffffb530, subject=subject@entry=0x7ffff4620630, limit=limit@entry=18446744073709551615, replace_count=0x7fffffffb498, regex=<optimized out>, regex=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2145
#29 0x000055555571c487 in preg_replace_func_impl (regex=regex@entry=0x7ffff4620610, fci=fci@entry=0x7fffffffb550, fcc=fcc@entry=0x7fffffffb530, subject=subject@entry=0x7ffff4620630, limit_val=-1, return_value=<optimized out>, return_value=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2195
#30 0x000055555571c91e in zif_preg_replace_callback (execute_data=0x7ffff46205c0, return_value=0x7ffff4620580) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/ext/pcre/php_pcre.c:2358
#31 0x0000555555a7912f in ZEND_DO_ICALL_SPEC_RETVAL_USED_HANDLER () at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:690
#32 execute_ex (ex=0x7ffff4600040) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:55465
#33 0x0000555555a7d062 in zend_execute (op_array=op_array@entry=0x7ffff46750e0, return_value=0x0, return_value@entry=0x7fffb3d80b90) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend_vm_execute.h:60881
#34 0x00005555559f6944 in zend_execute_scripts (type=type@entry=8, retval=0x7fffb3d80b90, retval@entry=0x0, file_count=-194902832, file_count@entry=3) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/Zend/zend.c:1568
#35 0x000055555599b298 in php_execute_script (primary_file=0x7fffffffdc40) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/main/main.c:2630
#36 0x00005555556d8bd9 in main (argc=<optimized out>, argv=<optimized out>) at /var/tmp/portage/dev-lang/php-7.3.7/work/sapis-build/fpm/sapi/fpm/fpm/fpm_main.c:1950
(gdb)


It's happening on a WordPress site using "WPBakery Page Builder" plugin. It crashes when a theme calls "vc_map()" function. It doesn't crash when opcode is disabled (opcache.enable=0).

PHP-7.3.6 works, PHP-7.3.7 shows this error.