| .TH PCREPRECOMPILE 3 |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "SAVING AND RE-USING PRECOMPILED PCRE PATTERNS" |
| .rs |
| .sp |
| If you are running an application that uses a large number of regular |
| expression patterns, it may be useful to store them in a precompiled form |
| instead of having to compile them every time the application is run. |
| If you are not using any private character tables (see the |
| .\" HREF |
| \fBpcre_maketables()\fP |
| .\" |
| documentation), this is relatively straightforward. If you are using private |
| tables, it is a little bit more complicated. However, if you are using the |
| just-in-time optimization feature of \fBpcre_study()\fP, it is not possible to |
| save and reload the JIT data. |
| .P |
| If you save compiled patterns to a file, you can copy them to a different host |
| and run them there. This works even if the new host has the opposite endianness |
| to the one on which the patterns were compiled. There may be a small |
| performance penalty, but it should be insignificant. However, compiling regular |
| expressions with one version of PCRE for use with a different version is not |
| guaranteed to work and may cause crashes, and saving and restoring a compiled |
| pattern loses any JIT optimization data. |
| . |
| . |
| .SH "SAVING A COMPILED PATTERN" |
| .rs |
| .sp |
| The value returned by \fBpcre_compile()\fP points to a single block of memory |
| that holds the compiled pattern and associated data. You can find the length of |
| this block in bytes by calling \fBpcre_fullinfo()\fP with an argument of |
| PCRE_INFO_SIZE. You can then save the data in any appropriate manner. Here is |
| sample code that compiles a pattern and writes it to a file. It assumes that |
| the variable \fIfd\fP refers to a file that is open for output: |
| .sp |
| int erroroffset, rc, size; |
| char *error; |
| pcre *re; |
| .sp |
| re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); |
| if (re == NULL) { ... handle errors ... } |
| rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); |
| if (rc < 0) { ... handle errors ... } |
| rc = fwrite(re, 1, size, fd); |
| if (rc != size) { ... handle errors ... } |
| .sp |
| In this example, the bytes that comprise the compiled pattern are copied |
| exactly. Note that this is binary data that may contain any of the 256 possible |
| byte values. On systems that make a distinction between binary and non-binary |
| data, be sure that the file is opened for binary output. |
| .P |
| If you want to write more than one pattern to a file, you will have to devise a |
| way of separating them. For binary data, preceding each pattern with its length |
| is probably the most straightforward approach. Another possibility is to write |
| out the data in hexadecimal instead of binary, one pattern to a line. |
| .P |
| Saving compiled patterns in a file is only one possible way of storing them for |
| later use. They could equally well be saved in a database, or in the memory of |
| some daemon process that passes them via sockets to the processes that want |
| them. |
| .P |
| If the pattern has been studied, it is also possible to save the normal study |
| data in a similar way to the compiled pattern itself. However, if the |
| PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is created cannot |
| be saved because it is too dependent on the current environment. When studying |
| generates additional information, \fBpcre_study()\fP returns a pointer to a |
| \fBpcre_extra\fP data block. Its format is defined in the |
| .\" HTML <a href="pcreapi.html#extradata"> |
| .\" </a> |
| section on matching a pattern |
| .\" |
| in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| documentation. The \fIstudy_data\fP field points to the binary study data, and |
| this is what you must save (not the \fBpcre_extra\fP block itself). The length |
| of the study data can be obtained by calling \fBpcre_fullinfo()\fP with an |
| argument of PCRE_INFO_STUDYSIZE. Remember to check that \fBpcre_study()\fP did |
| return a non-NULL value before trying to save the study data. |
| . |
| . |
| .SH "RE-USING A PRECOMPILED PATTERN" |
| .rs |
| .sp |
| Re-using a precompiled pattern is straightforward. Having reloaded it into main |
| memory, you pass its pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in |
| the usual way. This should work even on another host, and even if that host has |
| the opposite endianness to the one where the pattern was compiled. |
| .P |
| However, if you passed a pointer to custom character tables when the pattern |
| was compiled (the \fItableptr\fP argument of \fBpcre_compile()\fP), you must |
| now pass a similar pointer to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP, |
| because the value saved with the compiled pattern will obviously be nonsense. A |
| field in a \fBpcre_extra()\fP block is used to pass this data, as described in |
| the |
| .\" HTML <a href="pcreapi.html#extradata"> |
| .\" </a> |
| section on matching a pattern |
| .\" |
| in the |
| .\" HREF |
| \fBpcreapi\fP |
| .\" |
| documentation. |
| .P |
| If you did not provide custom character tables when the pattern was compiled, |
| the pointer in the compiled pattern is NULL, which causes \fBpcre_exec()\fP to |
| use PCRE's internal tables. Thus, you do not need to take any special action at |
| run time in this case. |
| .P |
| If you saved study data with the compiled pattern, you need to create your own |
| \fBpcre_extra\fP data block and set the \fIstudy_data\fP field to point to the |
| reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA bit in the |
| \fIflags\fP field to indicate that study data is present. Then pass the |
| \fBpcre_extra\fP block to \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP in the |
| usual way. If the pattern was studied for just-in-time optimization, that data |
| cannot be saved, and so is lost by a save/restore cycle. |
| . |
| . |
| .SH "COMPATIBILITY WITH DIFFERENT PCRE RELEASES" |
| .rs |
| .sp |
| In general, it is safest to recompile all saved patterns when you update to a |
| new PCRE release, though not all updates actually require this. |
| . |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 26 August 2011 |
| Copyright (c) 1997-2011 University of Cambridge. |
| .fi |