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<html><head><title>pcre2partial specification</title></head><body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB"><h1>pcre2partial man page</h1><p>Return to the <a href="index.html">PCRE2 index page</a>.</p><p>This page is part of the PCRE2 HTML documentation. It was generatedautomatically from the original man page. If there is any nonsense in it,please consult the man page, in case the conversion went wrong.<br><ul><li><a name="TOC1" href="#SEC1">PARTIAL MATCHING IN PCRE2</a><li><a name="TOC2" href="#SEC2">REQUIREMENTS FOR A PARTIAL MATCH</a><li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre2_match()</a><li><a name="TOC4" href="#SEC4">MULTI-SEGMENT MATCHING WITH pcre2_match()</a><li><a name="TOC5" href="#SEC5">PARTIAL MATCHING USING pcre2_dfa_match()</a><li><a name="TOC6" href="#SEC6">MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()</a><li><a name="TOC7" href="#SEC7">AUTHOR</a><li><a name="TOC8" href="#SEC8">REVISION</a></ul><br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE2</a><br><P>In normal use of PCRE2, if there is a match up to the end of a subject string,but more characters are needed to match the entire pattern, PCRE2_ERROR_NOMATCHis returned, just like any other failing match. There are circumstances whereit might be helpful to distinguish this "partial match" case.</P><P>One example is an application where the subject string is very long, and notall available at once. The requirement here is to be able to do the matchingsegment by segment, but special action is needed when a matched substring spansthe boundary between two segments.</P><P>Another example is checking a user input string as it is typed, to ensure thatit conforms to a required format. Invalid characters can be immediatelydiagnosed and rejected, giving instant feedback.</P><P>Partial matching is a PCRE2-specific feature; it is not Perl-compatible. It isrequested by setting one of the PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFToptions when calling a matching function. The difference between the twooptions is whether or not a partial match is preferred to an alternativecomplete match, though the details differ between the two types of matchingfunction. If both options are set, PCRE2_PARTIAL_HARD takes precedence.</P><P>If you want to use partial matching with just-in-time optimized code, as wellas setting a partial match option for the matching function, you must also call<b>pcre2_jit_compile()</b> with one or both of these options:<pre> PCRE2_JIT_PARTIAL_HARD PCRE2_JIT_PARTIAL_SOFT</pre>PCRE2_JIT_COMPLETE should also be set if you are going to run non-partialmatches on the same pattern. Separate code is compiled for each mode. If theappropriate JIT mode has not been compiled, interpretive matching code is used.</P><P>Setting a partial matching option disables two of PCRE2's standardoptimization hints. PCRE2 remembers the last literal code unit in a pattern,and abandons matching immediately if it is not present in the subject string.This optimization cannot be used for a subject string that might match onlypartially. PCRE2 also remembers a minimum length of a matching string, and doesnot bother to run the matching function on shorter strings. This optimizationis also disabled for partial matching.</P><br><a name="SEC2" href="#TOC1">REQUIREMENTS FOR A PARTIAL MATCH</a><br><P>A possible partial match occurs during matching when the end of the subjectstring is reached successfully, but either more characters are needed tocomplete the match, or the addition of more characters might change what ismatched.</P><P>Example 1: if the pattern is /abc/ and the subject is "ab", more characters aredefinitely needed to complete a match. In this case both hard and soft matchingoptions yield a partial match.</P><P>Example 2: if the pattern is /ab+/ and the subject is "ab", a complete matchcan be found, but the addition of more characters might change what ismatched. In this case, only PCRE2_PARTIAL_HARD returns a partial match;PCRE2_PARTIAL_SOFT returns the complete match.</P><P>On reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if the nextpattern item is \z, \Z, \b, \B, or $ there is always a partial match.Otherwise, for both options, the next pattern item must be one that inspects acharacter, and at least one of the following must be true:</P><P>(1) At least one character has already been inspected. An inspected characterneed not form part of the final matched string; lookbehind assertions and the\K escape sequence provide ways of inspecting characters before the start of amatched string.</P><P>(2) The pattern contains one or more lookbehind assertions. This conditionexists in case there is a lookbehind that inspects characters before the startof the match.</P><P>(3) There is a special case when the whole pattern can match an empty string.When the starting point is at the end of the subject, the empty string match isa possibility, and if PCRE2_PARTIAL_SOFT is set and neither of the aboveconditions is true, it is returned. However, because adding more charactersmight result in a non-empty match, PCRE2_PARTIAL_HARD returns a partial match,which in this case means "there is going to be a match at this point, but untilsome more characters are added, we do not know if it will be an empty string orsomething longer".</P><br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre2_match()</a><br><P>When a partial matching option is set, the result of calling<b>pcre2_match()</b> can be one of the following:</P><P><b>A successful match</b>A complete match has been found, starting and ending within this subject.</P><P><b>PCRE2_ERROR_NOMATCH</b>No match can start anywhere in this subject.</P><P><b>PCRE2_ERROR_PARTIAL</b>Adding more characters may result in a complete match that uses one or morecharacters from the end of this subject.</P><P>When a partial match is returned, the first two elements in the ovector pointto the portion of the subject that was matched, but the values in the rest ofthe ovector are undefined. The appearance of \K in the pattern has no effectfor a partial match. Consider this pattern:<pre> /abc\K123/</pre>If it is matched against "456abc123xyz" the result is a complete match, and theovector defines the matched string as "123", because \K resets the "start ofmatch" point. However, if a partial match is requested and the subject stringis "456abc12", a partial match is found for the string "abc12", because allthese characters are needed for a subsequent re-match with additionalcharacters.</P><P>If there is more than one partial match, the first one that was found providesthe data that is returned. Consider this pattern:<pre> /123\w+X|dogY/</pre>If this is matched against the subject string "abc123dog", both alternativesfail to match, but the end of the subject is reached during matching, soPCRE2_ERROR_PARTIAL is returned. The offsets are set to 3 and 9, identifying"123dog" as the first partial match. (In this example, there are two partialmatches, because "dog" on its own partially matches the second alternative.)</P><br><b>How a partial match is processed by pcre2_match()</b><br><P>What happens when a partial match is identified depends on which of the twopartial matching options is set.</P><P>If PCRE2_PARTIAL_HARD is set, PCRE2_ERROR_PARTIAL is returned as soon as apartial match is found, without continuing to search for possible completematches. This option is "hard" because it prefers an earlier partial match overa later complete match. For this reason, the assumption is made that the end ofthe supplied subject string is not the true end of the available data, which iswhy \z, \Z, \b, \B, and $ always give a partial match.</P><P>If PCRE2_PARTIAL_SOFT is set, the partial match is remembered, but matchingcontinues as normal, and other alternatives in the pattern are tried. If nocomplete match can be found, PCRE2_ERROR_PARTIAL is returned instead ofPCRE2_ERROR_NOMATCH. This option is "soft" because it prefers a complete matchover a partial match. All the various matching items in a pattern behave as ifthe subject string is potentially complete; \z, \Z, and $ match at the end ofthe subject, as normal, and for \b and \B the end of the subject is treatedas a non-alphanumeric.</P><P>The difference between the two partial matching options can be illustrated by apattern such as:<pre> /dog(sbody)?/</pre>This matches either "dog" or "dogsbody", greedily (that is, it prefers thelonger string if possible). If it is matched against the string "dog" withPCRE2_PARTIAL_SOFT, it yields a complete match for "dog". However, ifPCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PARTIAL. On the otherhand, if the pattern is made ungreedy the result is different:<pre> /dog(sbody)??/</pre>In this case the result is always a complete match because that is found first,and matching never continues after finding a complete match. It might be easierto follow this explanation by thinking of the two patterns like this:<pre> /dog(sbody)?/ is the same as /dogsbody|dog/ /dog(sbody)??/ is the same as /dog|dogsbody/</pre>The second pattern will never match "dogsbody", because it will always find theshorter match first.</P><br><b>Example of partial matching using pcre2test</b><br><P>The <b>pcre2test</b> data modifiers <b>partial_hard</b> (or <b>ph</b>) and<b>partial_soft</b> (or <b>ps</b>) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT,respectively, when calling <b>pcre2_match()</b>. Here is a run of<b>pcre2test</b> using a pattern that matches the whole subject in the form of adate:<pre> re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25dec3\=ph Partial match: 23dec3 data> 3ju\=ph Partial match: 3ju data> 3juj\=ph No match</pre>This example gives the same results for both hard and soft partial matchingoptions. Here is an example where there is a difference:<pre> re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 25jun04\=ps 0: 25jun04 1: jun data> 25jun04\=ph Partial match: 25jun04</pre>With PCRE2_PARTIAL_SOFT, the subject is matched completely. ForPCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete, sothere is only a partial match.</P><br><a name="SEC4" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre2_match()</a><br><P>PCRE was not originally designed with multi-segment matching in mind. However,over time, features (including partial matching) that make multi-segmentmatching possible have been added. A very long string can be searched segmentby segment by calling <b>pcre2_match()</b> repeatedly, with the aim of achievingthe same results that would happen if the entire string was available forsearching all the time. Normally, the strings that are being sought are muchshorter than each individual segment, and are in the middle of very longstrings, so the pattern is normally not anchored.</P><P>Special logic must be implemented to handle a matched substring that spans asegment boundary. PCRE2_PARTIAL_HARD should be used, because it returns apartial match at the end of a segment whenever there is the possibility ofchanging the match by adding more characters. The PCRE2_NOTBOL option shouldalso be set for all but the first segment.</P><P>When a partial match occurs, the next segment must be added to the currentsubject and the match re-run, using the <i>startoffset</i> argument of<b>pcre2_match()</b> to begin at the point where the partial match started.For example:<pre> re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ data> ...the date is 23ja\=ph Partial match: 23ja data> ...the date is 23jan19 and on that day...\=offset=15 0: 23jan19 1: jan</pre>Note the use of the <b>offset</b> modifier to start the new match where thepartial match was found. In this example, the next segment was added to the onein which the partial match was found. This is the most straightforwardapproach, typically using a memory buffer that is twice the size of eachsegment. After a partial match, the first half of the buffer is discarded, thesecond half is moved to the start of the buffer, and a new segment is addedbefore repeating the match as in the example above. After a no match, theentire buffer can be discarded.</P><P>If there are memory constraints, you may want to discard text that precedes apartial match before adding the next segment. Unfortunately, this is not atpresent straightforward. In cases such as the above, where the pattern does notcontain any lookbehinds, it is sufficient to retain only the partially matchedsubstring. However, if the pattern contains a lookbehind assertion, charactersthat precede the start of the partial match may have been inspected during thematching process. When <b>pcre2test</b> displays a partial match, it indicatesthese characters with '<' if the <b>allusedtext</b> modifier is set:<pre> re> "(?<=123)abc" data> xx123ab\=ph,allusedtext Partial match: 123ab <<<</pre>However, the <b>allusedtext</b> modifier is not available for JIT matching,because JIT matching does not record the first (or last) consulted characters.For this reason, this information is not available via the API. It is thereforenot possible in general to obtain the exact number of characters that must beretained in order to get the right match result. If you cannot retain theentire segment, you must find some heuristic way of choosing.</P><P>If you know the approximate length of the matching substrings, you can use thatto decide how much text to retain. The only lookbehind information that iscurrently available via the API is the length of the longest individuallookbehind in a pattern, but this can be misleading if there are nestedlookbehinds. The value returned by calling <b>pcre2_pattern_info()</b> with thePCRE2_INFO_MAXLOOKBEHIND option is the maximum number of characters (not codeunits) that any individual lookbehind moves back when it is processed. Apattern such as "(?<=(?<!b)a)" has a maximum lookbehind value of one, butinspects two characters before its starting point.</P><P>In a non-UTF or a 32-bit case, moving back is just a subtraction, but inUTF-8 or UTF-16 you have to count characters while moving back through the codeunits.</P><br><a name="SEC5" href="#TOC1">PARTIAL MATCHING USING pcre2_dfa_match()</a><br><P>The DFA function moves along the subject string character by character, withoutbacktracking, searching for all possible matches simultaneously. If the end ofthe subject is reached before the end of the pattern, there is the possibilityof a partial match.</P><P>When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if therehave been no complete matches. Otherwise, the complete matches are returned.If PCRE2_PARTIAL_HARD is set, a partial match takes precedence over anycomplete matches. The portion of the string that was matched when the longestpartial match was found is set as the first matching string.</P><P>Because the DFA function always searches for all possible matches, and there isno difference between greedy and ungreedy repetition, its behaviour isdifferent from the <b>pcre2_match()</b>. Consider the string "dog" matchedagainst this ungreedy pattern:<pre> /dog(sbody)??/</pre>Whereas the standard function stops as soon as it finds the complete match for"dog", the DFA function also finds the partial match for "dogsbody", and soreturns that when PCRE2_PARTIAL_HARD is set.</P><br><a name="SEC6" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()</a><br><P>When a partial match has been found using the DFA matching function, it ispossible to continue the match by providing additional subject data and callingthe function again with the same compiled regular expression, this time settingthe PCRE2_DFA_RESTART option. You must pass the same working space as before,because this is where details of the previous partial match are stored. You canset the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTARTto continue partial matching over multiple segments. Here is an example using<b>pcre2test</b>:<pre> re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ data> 23ja\=dfa,ps Partial match: 23ja data> n05\=dfa,dfa_restart 0: n05</pre>The first call has "23ja" as the subject, and requests partial matching; thesecond call has "n05" as the subject for the continued (restarted) match.Notice that when the match is complete, only the last part is shown; PCRE2 doesnot retain the previously partially-matched string. It is up to the callingprogram to do that if it needs to. This means that, for an unanchored pattern,if a continued match fails, it is not possible to try again at a new startingpoint. All this facility is capable of doing is continuing with the previousmatch attempt. For example, consider this pattern:<pre> 1234|3789</pre>If the first part of the subject is "ABC123", a partial match of the firstalternative is found at offset 3. There is no partial match for the secondalternative, because such a match does not start at the same point in thesubject string. Attempting to continue with the string "7890" does not yield amatch because only those alternatives that match at one point in the subjectare remembered. Depending on the application, this may or may not be what youwant.</P><P>If you do want to allow for starting again at the next character, one way ofdoing it is to retain some or all of the segment and try a new complete match,as described for <b>pcre2_match()</b> above. Another possibility is to work withtwo buffers. If a partial match at offset <i>n</i> in the first buffer isfollowed by "no match" when PCRE2_DFA_RESTART is used on the second buffer, youcan then try a new match starting at offset <i>n+1</i> in the first buffer.</P><br><a name="SEC7" href="#TOC1">AUTHOR</a><br><P>Philip Hazel<br>Retired from University Computing Service<br>Cambridge, England.<br></P><br><a name="SEC8" href="#TOC1">REVISION</a><br><P>Last updated: 04 September 2019<br>Copyright © 1997-2019 University of Cambridge.<br><p>Return to the <a href="index.html">PCRE2 index page</a>.</p>
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