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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 PCRE</a> +<li><a name="TOC2" href="#SEC2">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a> +<li><a name="TOC3" href="#SEC3">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a> +<li><a name="TOC4" href="#SEC4">PARTIAL MATCHING AND WORD BOUNDARIES</a> +<li><a name="TOC5" href="#SEC5">FORMERLY RESTRICTED PATTERNS</a> +<li><a name="TOC6" href="#SEC6">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a> +<li><a name="TOC7" href="#SEC7">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a> +<li><a name="TOC8" href="#SEC8">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a> +<li><a name="TOC9" href="#SEC9">ISSUES WITH MULTI-SEGMENT MATCHING</a> +<li><a name="TOC10" href="#SEC10">AUTHOR</a> +<li><a name="TOC11" href="#SEC11">REVISION</a> +</ul> +<br><a name="SEC1" href="#TOC1">PARTIAL MATCHING IN PCRE</a><br> +<P> +In normal use of PCRE, if the subject string that is passed to a matching +function matches as far as it goes, but is too short to match the entire +pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances where it might +be helpful to distinguish this case from other cases in which there is no +match. +</P> +<P> +Consider, for example, an application where a human is required to type in data +for a field with specific formatting requirements. An example might be a date +in the form <i>ddmmmyy</i>, defined by this pattern: +<pre> + ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ +</pre> +If the application sees the user's keystrokes one by one, and can check that +what has been typed so far is potentially valid, it is able to raise an error +as soon as a mistake is made, by beeping and not reflecting the character that +has been typed, for example. This immediate feedback is likely to be a better +user interface than a check that is delayed until the entire string has been +entered. Partial matching can also be useful when the subject string is very +long and is not all available at once. +</P> +<P> +PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and +PCRE_PARTIAL_HARD options, which can be set when calling any of the matching +functions. For backwards compatibility, PCRE_PARTIAL is a synonym for +PCRE_PARTIAL_SOFT. The essential difference between the two options is whether +or not a partial match is preferred to an alternative complete match, though +the details differ between the two types of matching function. If both options +are set, PCRE_PARTIAL_HARD takes precedence. +</P> +<P> +If you want to use partial matching with just-in-time optimized code, you must +call <b>pcre_study()</b>, <b>pcre16_study()</b> or <b>pcre32_study()</b> with one +or both of these options: +<pre> + PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE + PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE +</pre> +PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non-partial +matches on the same pattern. If the appropriate JIT study mode has not been set +for a match, the interpretive matching code is used. +</P> +<P> +Setting a partial matching option disables two of PCRE's standard +optimizations. PCRE remembers the last literal data 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 only +partially. If the pattern was studied, PCRE knows the minimum length of a +matching string, and does not bother to run the matching function on shorter +strings. This optimization is also disabled for partial matching. +</P> +<br><a name="SEC2" href="#TOC1">PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()</a><br> +<P> +A partial match occurs during a call to <b>pcre_exec()</b> or +<b>pcre[16|32]_exec()</b> when the end of the subject string is reached +successfully, but matching cannot continue because more characters are needed. +However, at least one character in the subject must have been inspected. This +character need not form part of the final matched string; lookbehind assertions +and the \K escape sequence provide ways of inspecting characters before the +start of a matched substring. The requirement for inspecting at least one +character exists because an empty string can always be matched; without such a +restriction there would always be a partial match of an empty string at the end +of the subject. +</P> +<P> +If there are at least two slots in the offsets vector when a partial match is +returned, the first slot is set to the offset of the earliest character that +was inspected. For convenience, the second offset points to the end of the +subject so that a substring can easily be identified. If there are at least +three slots in the offsets vector, the third slot is set to the offset of the +character where matching started. +</P> +<P> +For the majority of patterns, the contents of the first and third slots will be +the same. However, for patterns that contain lookbehind assertions, or begin +with \b or \B, characters before the one where matching started may have been +inspected while carrying out the match. For example, consider this pattern: +<pre> + /(?<=abc)123/ +</pre> +This pattern matches "123", but only if it is preceded by "abc". If the subject +string is "xyzabc12", the first two offsets after a partial match are for the +substring "abc12", because all these characters were inspected. However, the +third offset is set to 6, because that is the offset where matching began. +</P> +<P> +What happens when a partial match is identified depends on which of the two +partial matching options are set. +</P> +<br><b> +PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec() +</b><br> +<P> +If PCRE_PARTIAL_SOFT is set when <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b> +identifies a partial match, the partial match is remembered, but matching +continues as normal, and other alternatives in the pattern are tried. If no +complete match can be found, PCRE_ERROR_PARTIAL is returned instead of +PCRE_ERROR_NOMATCH. +</P> +<P> +This option is "soft" because it prefers a complete match over a partial match. +All the various matching items in a pattern behave as if the subject string is +potentially complete. For example, \z, \Z, and $ match at the end of the +subject, as normal, and for \b and \B the end of the subject is treated as a +non-alphanumeric. +</P> +<P> +If there is more than one partial match, the first one that was found provides +the data that is returned. Consider this pattern: +<pre> + /123\w+X|dogY/ +</pre> +If this is matched against the subject string "abc123dog", both +alternatives fail to match, but the end of the subject is reached during +matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 and 9, +identifying "123dog" as the first partial match that was found. (In this +example, there are two partial matches, because "dog" on its own partially +matches the second alternative.) +</P> +<br><b> +PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec() +</b><br> +<P> +If PCRE_PARTIAL_HARD is set for <b>pcre_exec()</b> or <b>pcre[16|32]_exec()</b>, +PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, without +continuing to search for possible complete matches. This option is "hard" +because it prefers an earlier partial match over a later complete match. For +this reason, the assumption is made that the end of the supplied subject string +may not be the true end of the available data, and so, if \z, \Z, \b, \B, +or $ are encountered at the end of the subject, the result is +PCRE_ERROR_PARTIAL, provided that at least one character in the subject has +been inspected. +</P> +<P> +Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 +subject strings are checked for validity. Normally, an invalid sequence +causes the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the +special case of a truncated character at the end of the subject, +PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when +PCRE_PARTIAL_HARD is set. +</P> +<br><b> +Comparing hard and soft partial matching +</b><br> +<P> +The difference between the two partial matching options can be illustrated by a +pattern such as: +<pre> + /dog(sbody)?/ +</pre> +This matches either "dog" or "dogsbody", greedily (that is, it prefers the +longer string if possible). If it is matched against the string "dog" with +PCRE_PARTIAL_SOFT, it yields a complete match for "dog". However, if +PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. On the other hand, +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 easier +to 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 the +shorter match first. +</P> +<br><a name="SEC3" href="#TOC1">PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br> +<P> +The DFA functions move along the subject string character by character, without +backtracking, searching for all possible matches simultaneously. If the end of +the subject is reached before the end of the pattern, there is the possibility +of a partial match, again provided that at least one character has been +inspected. +</P> +<P> +When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if there +have been no complete matches. Otherwise, the complete matches are returned. +However, if PCRE_PARTIAL_HARD is set, a partial match takes precedence over any +complete matches. The portion of the string that was inspected when the longest +partial match was found is set as the first matching string, provided there are +at least two slots in the offsets vector. +</P> +<P> +Because the DFA functions always search for all possible matches, and there is +no difference between greedy and ungreedy repetition, their behaviour is +different from the standard functions when PCRE_PARTIAL_HARD is set. Consider +the string "dog" matched against the ungreedy pattern shown above: +<pre> + /dog(sbody)??/ +</pre> +Whereas the standard functions stop as soon as they find the complete match for +"dog", the DFA functions also find the partial match for "dogsbody", and so +return that when PCRE_PARTIAL_HARD is set. +</P> +<br><a name="SEC4" href="#TOC1">PARTIAL MATCHING AND WORD BOUNDARIES</a><br> +<P> +If a pattern ends with one of sequences \b or \B, which test for word +boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive +results. Consider this pattern: +<pre> + /\bcat\b/ +</pre> +This matches "cat", provided there is a word boundary at either end. If the +subject string is "the cat", the comparison of the final "t" with a following +character cannot take place, so a partial match is found. However, normal +matching carries on, and \b matches at the end of the subject when the last +character is a letter, so a complete match is found. The result, therefore, is +<i>not</i> PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield +PCRE_ERROR_PARTIAL, because then the partial match takes precedence. +</P> +<br><a name="SEC5" href="#TOC1">FORMERLY RESTRICTED PATTERNS</a><br> +<P> +For releases of PCRE prior to 8.00, because of the way certain internal +optimizations were implemented in the <b>pcre_exec()</b> function, the +PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be used with +all patterns. From release 8.00 onwards, the restrictions no longer apply, and +partial matching with can be requested for any pattern. +</P> +<P> +Items that were formerly restricted were repeated single characters and +repeated metasequences. If PCRE_PARTIAL was set for a pattern that did not +conform to the restrictions, <b>pcre_exec()</b> returned the error code +PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The +PCRE_INFO_OKPARTIAL call to <b>pcre_fullinfo()</b> to find out if a compiled +pattern can be used for partial matching now always returns 1. +</P> +<br><a name="SEC6" href="#TOC1">EXAMPLE OF PARTIAL MATCHING USING PCRETEST</a><br> +<P> +If the escape sequence \P is present in a <b>pcretest</b> data line, the +PCRE_PARTIAL_SOFT option is used for the match. Here is a run of <b>pcretest</b> +that uses the date example quoted above: +<pre> + re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 25jun04\P + 0: 25jun04 + 1: jun + data> 25dec3\P + Partial match: 23dec3 + data> 3ju\P + Partial match: 3ju + data> 3juj\P + No match + data> j\P + No match +</pre> +The first data string is matched completely, so <b>pcretest</b> shows the +matched substrings. The remaining four strings do not match the complete +pattern, but the first two are partial matches. Similar output is obtained +if DFA matching is used. +</P> +<P> +If the escape sequence \P is present more than once in a <b>pcretest</b> data +line, the PCRE_PARTIAL_HARD option is set for the match. +</P> +<br><a name="SEC7" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()</a><br> +<P> +When a partial match has been found using a DFA matching function, it is +possible to continue the match by providing additional subject data and calling +the function again with the same compiled regular expression, this time setting +the PCRE_DFA_RESTART option. You must pass the same working space as before, +because this is where details of the previous partial match are stored. Here is +an example using <b>pcretest</b>, using the \R escape sequence to set the +PCRE_DFA_RESTART option (\D specifies the use of the DFA matching function): +<pre> + re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ + data> 23ja\P\D + Partial match: 23ja + data> n05\R\D + 0: n05 +</pre> +The first call has "23ja" as the subject, and requests partial matching; the +second call has "n05" as the subject for the continued (restarted) match. +Notice that when the match is complete, only the last part is shown; PCRE does +not retain the previously partially-matched string. It is up to the calling +program to do that if it needs to. +</P> +<P> +That means that, for an unanchored pattern, if a continued match fails, it is +not possible to try again at a new starting point. All this facility is capable +of doing is continuing with the previous match attempt. In the previous +example, if the second set of data is "ug23" the result is no match, even +though there would be a match for "aug23" if the entire string were given at +once. Depending on the application, this may or may not be what you want. +The only way to allow for starting again at the next character is to retain the +matched part of the subject and try a new complete match. +</P> +<P> +You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with +PCRE_DFA_RESTART to continue partial matching over multiple segments. This +facility can be used to pass very long subject strings to the DFA matching +functions. +</P> +<br><a name="SEC8" href="#TOC1">MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()</a><br> +<P> +From release 8.00, the standard matching functions can also be used to do +multi-segment matching. Unlike the DFA functions, it is not possible to +restart the previous match with a new segment of data. Instead, new data must +be added to the previous subject string, and the entire match re-run, starting +from the point where the partial match occurred. Earlier data can be discarded. +</P> +<P> +It is best to use PCRE_PARTIAL_HARD in this situation, because it does not +treat the end of a segment as the end of the subject when matching \z, \Z, +\b, \B, and $. Consider an unanchored pattern that matches dates: +<pre> + re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ + data> The date is 23ja\P\P + Partial match: 23ja +</pre> +At this stage, an application could discard the text preceding "23ja", add on +text from the next segment, and call the matching function again. Unlike the +DFA matching functions, the entire matching string must always be available, +and the complete matching process occurs for each call, so more memory and more +processing time is needed. +</P> +<P> +<b>Note:</b> If the pattern contains lookbehind assertions, or \K, or starts +with \b or \B, the string that is returned for a partial match includes +characters that precede the start of what would be returned for a complete +match, because it contains all the characters that were inspected during the +partial match. +</P> +<br><a name="SEC9" href="#TOC1">ISSUES WITH MULTI-SEGMENT MATCHING</a><br> +<P> +Certain types of pattern may give problems with multi-segment matching, +whichever matching function is used. +</P> +<P> +1. If the pattern contains a test for the beginning of a line, you need to pass +the PCRE_NOTBOL option when the subject string for any call does start at the +beginning of a line. There is also a PCRE_NOTEOL option, but in practice when +doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which +includes the effect of PCRE_NOTEOL. +</P> +<P> +2. Lookbehind assertions that have already been obeyed are catered for in the +offsets that are returned for a partial match. However a lookbehind assertion +later in the pattern could require even earlier characters to be inspected. You +can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the +<b>pcre_fullinfo()</b> or <b>pcre[16|32]_fullinfo()</b> functions to obtain the +length of the longest lookbehind in the pattern. This length is given in +characters, not bytes. If you always retain at least that many characters +before the partially matched string, all should be well. (Of course, near the +start of the subject, fewer characters may be present; in that case all +characters should be retained.) +</P> +<P> +From release 8.33, there is a more accurate way of deciding which characters to +retain. Instead of subtracting the length of the longest lookbehind from the +earliest inspected character (<i>offsets[0]</i>), the match start position +(<i>offsets[2]</i>) should be used, and the next match attempt started at the +<i>offsets[2]</i> character by setting the <i>startoffset</i> argument of +<b>pcre_exec()</b> or <b>pcre_dfa_exec()</b>. +</P> +<P> +For example, if the pattern "(?<=123)abc" is partially +matched against the string "xx123a", the three offset values returned are 2, 6, +and 5. This indicates that the matching process that gave a partial match +started at offset 5, but the characters "123a" were all inspected. The maximum +lookbehind for that pattern is 3, so taking that away from 5 shows that we need +only keep "123a", and the next match attempt can be started at offset 3 (that +is, at "a") when further characters have been added. When the match start is +not the earliest inspected character, <b>pcretest</b> shows it explicitly: +<pre> + re> "(?<=123)abc" + data> xx123a\P\P + Partial match at offset 5: 123a +</PRE> +</P> +<P> +3. Because a partial match must always contain at least one character, what +might be considered a partial match of an empty string actually gives a "no +match" result. For example: +<pre> + re> /c(?<=abc)x/ + data> ab\P + No match +</pre> +If the next segment begins "cx", a match should be found, but this will only +happen if characters from the previous segment are retained. For this reason, a +"no match" result should be interpreted as "partial match of an empty string" +when the pattern contains lookbehinds. +</P> +<P> +4. Matching a subject string that is split into multiple segments may not +always produce exactly the same result as matching over one single long string, +especially when PCRE_PARTIAL_SOFT is used. The section "Partial Matching and +Word Boundaries" above describes an issue that arises if the pattern ends with +\b or \B. Another kind of difference may occur when there are multiple +matching possibilities, because (for PCRE_PARTIAL_SOFT) a partial match result +is given only when there are no completed matches. This means that as soon as +the shortest match has been found, continuation to a new subject segment is no +longer possible. Consider again this <b>pcretest</b> example: +<pre> + re> /dog(sbody)?/ + data> dogsb\P + 0: dog + data> do\P\D + Partial match: do + data> gsb\R\P\D + 0: g + data> dogsbody\D + 0: dogsbody + 1: dog +</pre> +The first data line passes the string "dogsb" to a standard matching function, +setting the PCRE_PARTIAL_SOFT option. Although the string is a partial match +for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the shorter +string "dog" is a complete match. Similarly, when the subject is presented to +a DFA matching function in several parts ("do" and "gsb" being the first two) +the match stops when "dog" has been found, and it is not possible to continue. +On the other hand, if "dogsbody" is presented as a single string, a DFA +matching function finds both matches. +</P> +<P> +Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching +multi-segment data. The example above then behaves differently: +<pre> + re> /dog(sbody)?/ + data> dogsb\P\P + Partial match: dogsb + data> do\P\D + Partial match: do + data> gsb\R\P\P\D + Partial match: gsb +</pre> +5. Patterns that contain alternatives at the top level which do not all start +with the same pattern item may not work as expected when PCRE_DFA_RESTART is +used. For example, consider this pattern: +<pre> + 1234|3789 +</pre> +If the first part of the subject is "ABC123", a partial match of the first +alternative is found at offset 3. There is no partial match for the second +alternative, because such a match does not start at the same point in the +subject string. Attempting to continue with the string "7890" does not yield a +match because only those alternatives that match at one point in the subject +are remembered. The problem arises because the start of the second alternative +matches within the first alternative. There is no problem with anchored +patterns or patterns such as: +<pre> + 1234|ABCD +</pre> +where no string can be a partial match for both alternatives. This is not a +problem if a standard matching function is used, because the entire match has +to be rerun each time: +<pre> + re> /1234|3789/ + data> ABC123\P\P + Partial match: 123 + data> 1237890 + 0: 3789 +</pre> +Of course, instead of using PCRE_DFA_RESTART, the same technique of re-running +the entire match can also be used with the DFA matching functions. Another +possibility is to work with two buffers. If a partial match at offset <i>n</i> +in the first buffer is followed by "no match" when PCRE_DFA_RESTART is used on +the second buffer, you can then try a new match starting at offset <i>n+1</i> in +the first buffer. +</P> +<br><a name="SEC10" href="#TOC1">AUTHOR</a><br> +<P> +Philip Hazel +<br> +University Computing Service +<br> +Cambridge CB2 3QH, England. +<br> +</P> +<br><a name="SEC11" href="#TOC1">REVISION</a><br> +<P> +Last updated: 02 July 2013 +<br> +Copyright © 1997-2013 University of Cambridge. +<br> +<p> +Return to the <a href="index.html">PCRE index page</a>. +</p> |