| .TH PCREPARTIAL 3 "02 July 2013" "PCRE 8.34" |
| .SH NAME |
| PCRE - Perl-compatible regular expressions |
| .SH "PARTIAL MATCHING IN PCRE" |
| .rs |
| .sp |
| 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 |
| 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 \fIddmmmyy\fP, defined by this pattern: |
| .sp |
| ^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$ |
| .sp |
| 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 |
| 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 |
| If you want to use partial matching with just-in-time optimized code, you must |
| call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one |
| or both of these options: |
| .sp |
| PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE |
| PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE |
| .sp |
| 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 |
| 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. |
| . |
| . |
| .SH "PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()" |
| .rs |
| .sp |
| A partial match occurs during a call to \fBpcre_exec()\fP or |
| \fBpcre[16|32]_exec()\fP 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 \eK 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 |
| 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 |
| 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 \eb or \eB, characters before the one where matching started may have been |
| inspected while carrying out the match. For example, consider this pattern: |
| .sp |
| /(?<=abc)123/ |
| .sp |
| 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 |
| What happens when a partial match is identified depends on which of the two |
| partial matching options are set. |
| . |
| . |
| .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()" |
| .rs |
| .sp |
| If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP |
| 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 |
| 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, \ez, \eZ, and $ match at the end of the |
| subject, as normal, and for \eb and \eB the end of the subject is treated as a |
| non-alphanumeric. |
| .P |
| If there is more than one partial match, the first one that was found provides |
| the data that is returned. Consider this pattern: |
| .sp |
| /123\ew+X|dogY/ |
| .sp |
| 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.) |
| . |
| . |
| .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()" |
| .rs |
| .sp |
| If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre[16|32]_exec()\fP, |
| 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 \ez, \eZ, \eb, \eB, |
| 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 |
| 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. |
| . |
| . |
| .SS "Comparing hard and soft partial matching" |
| .rs |
| .sp |
| The difference between the two partial matching options can be illustrated by a |
| pattern such as: |
| .sp |
| /dog(sbody)?/ |
| .sp |
| 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: |
| .sp |
| /dog(sbody)??/ |
| .sp |
| 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: |
| .sp |
| /dog(sbody)?/ is the same as /dogsbody|dog/ |
| /dog(sbody)??/ is the same as /dog|dogsbody/ |
| .sp |
| The second pattern will never match "dogsbody", because it will always find the |
| shorter match first. |
| . |
| . |
| .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()" |
| .rs |
| .sp |
| 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 |
| 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 |
| 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: |
| .sp |
| /dog(sbody)??/ |
| .sp |
| 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. |
| . |
| . |
| .SH "PARTIAL MATCHING AND WORD BOUNDARIES" |
| .rs |
| .sp |
| If a pattern ends with one of sequences \eb or \eB, which test for word |
| boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-intuitive |
| results. Consider this pattern: |
| .sp |
| /\ebcat\eb/ |
| .sp |
| 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 \eb matches at the end of the subject when the last |
| character is a letter, so a complete match is found. The result, therefore, is |
| \fInot\fP PCRE_ERROR_PARTIAL. Using PCRE_PARTIAL_HARD in this case does yield |
| PCRE_ERROR_PARTIAL, because then the partial match takes precedence. |
| . |
| . |
| .SH "FORMERLY RESTRICTED PATTERNS" |
| .rs |
| .sp |
| For releases of PCRE prior to 8.00, because of the way certain internal |
| optimizations were implemented in the \fBpcre_exec()\fP 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 |
| 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, \fBpcre_exec()\fP returned the error code |
| PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The |
| PCRE_INFO_OKPARTIAL call to \fBpcre_fullinfo()\fP to find out if a compiled |
| pattern can be used for partial matching now always returns 1. |
| . |
| . |
| .SH "EXAMPLE OF PARTIAL MATCHING USING PCRETEST" |
| .rs |
| .sp |
| If the escape sequence \eP is present in a \fBpcretest\fP data line, the |
| PCRE_PARTIAL_SOFT option is used for the match. Here is a run of \fBpcretest\fP |
| that uses the date example quoted above: |
| .sp |
| re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/ |
| data> 25jun04\eP |
| 0: 25jun04 |
| 1: jun |
| data> 25dec3\eP |
| Partial match: 23dec3 |
| data> 3ju\eP |
| Partial match: 3ju |
| data> 3juj\eP |
| No match |
| data> j\eP |
| No match |
| .sp |
| The first data string is matched completely, so \fBpcretest\fP shows the |
| matched substrings. The remaining four strings do not match the complete |
| pattern, but the first two are partial matches. Similar output is obtained |
| if DFA matching is used. |
| .P |
| If the escape sequence \eP is present more than once in a \fBpcretest\fP data |
| line, the PCRE_PARTIAL_HARD option is set for the match. |
| . |
| . |
| .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()" |
| .rs |
| .sp |
| When a partial match has been found using a DFA matching function, it is |
| possible to continue the match by providing additional subject data and calling |
| the function again with the same compiled regular expression, this time setting |
| the PCRE_DFA_RESTART option. You must pass the same working space as before, |
| because this is where details of the previous partial match are stored. Here is |
| an example using \fBpcretest\fP, using the \eR escape sequence to set the |
| PCRE_DFA_RESTART option (\eD specifies the use of the DFA matching function): |
| .sp |
| re> /^\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed$/ |
| data> 23ja\eP\eD |
| Partial match: 23ja |
| data> n05\eR\eD |
| 0: n05 |
| .sp |
| 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 |
| 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 |
| 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. |
| . |
| . |
| .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()" |
| .rs |
| .sp |
| 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 |
| 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 \ez, \eZ, |
| \eb, \eB, and $. Consider an unanchored pattern that matches dates: |
| .sp |
| re> /\ed?\ed(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\ed\ed/ |
| data> The date is 23ja\eP\eP |
| Partial match: 23ja |
| .sp |
| 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 |
| \fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts |
| with \eb or \eB, 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. |
| . |
| . |
| .SH "ISSUES WITH MULTI-SEGMENT MATCHING" |
| .rs |
| .sp |
| Certain types of pattern may give problems with multi-segment matching, |
| whichever matching function is used. |
| .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 |
| 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 |
| \fBpcre_fullinfo()\fP or \fBpcre[16|32]_fullinfo()\fP 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 |
| 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 (\fIoffsets[0]\fP), the match start position |
| (\fIoffsets[2]\fP) should be used, and the next match attempt started at the |
| \fIoffsets[2]\fP character by setting the \fIstartoffset\fP argument of |
| \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. |
| .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, \fBpcretest\fP shows it explicitly: |
| .sp |
| re> "(?<=123)abc" |
| data> xx123a\eP\eP |
| Partial match at offset 5: 123a |
| .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: |
| .sp |
| re> /c(?<=abc)x/ |
| data> ab\eP |
| No match |
| .sp |
| 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 |
| 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 |
| \eb or \eB. 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 \fBpcretest\fP example: |
| .sp |
| re> /dog(sbody)?/ |
| data> dogsb\eP |
| 0: dog |
| data> do\eP\eD |
| Partial match: do |
| data> gsb\eR\eP\eD |
| 0: g |
| data> dogsbody\eD |
| 0: dogsbody |
| 1: dog |
| .sp |
| 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 |
| Because of these problems, it is best to use PCRE_PARTIAL_HARD when matching |
| multi-segment data. The example above then behaves differently: |
| .sp |
| re> /dog(sbody)?/ |
| data> dogsb\eP\eP |
| Partial match: dogsb |
| data> do\eP\eD |
| Partial match: do |
| data> gsb\eR\eP\eP\eD |
| Partial match: gsb |
| .sp |
| 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: |
| .sp |
| 1234|3789 |
| .sp |
| 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: |
| .sp |
| 1234|ABCD |
| .sp |
| 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: |
| .sp |
| re> /1234|3789/ |
| data> ABC123\eP\eP |
| Partial match: 123 |
| data> 1237890 |
| 0: 3789 |
| .sp |
| 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 \fIn\fP |
| 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 \fIn+1\fP in |
| the first buffer. |
| . |
| . |
| .SH AUTHOR |
| .rs |
| .sp |
| .nf |
| Philip Hazel |
| University Computing Service |
| Cambridge CB2 3QH, England. |
| .fi |
| . |
| . |
| .SH REVISION |
| .rs |
| .sp |
| .nf |
| Last updated: 02 July 2013 |
| Copyright (c) 1997-2013 University of Cambridge. |
| .fi |