NAME¶

PCRE2 - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE2¶

In normal use of PCRE2, 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, PCRE2_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.

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 ddmmmyy, defined by this pattern:

^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

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.

PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT and PCRE2_PARTIAL_HARD options, which can be set when calling a matching function. The 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, PCRE2_PARTIAL_HARD takes precedence.

If you want to use partial matching with just-in-time optimized code, you must call pcre2_jit_compile() with one or both of these options:

PCRE2_JIT_PARTIAL_SOFT
PCRE2_JIT_PARTIAL_HARD

PCRE2_JIT_COMPLETE should also be set if you are going to run non-partial matches on the same pattern. If the appropriate JIT mode has not been compiled, interpretive matching code is used.

Setting a partial matching option disables two of PCRE2's standard optimizations. 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 only partially. PCRE2 also 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.

PARTIAL MATCHING USING pcre2_match()¶

A partial match occurs during a call to pcre2_match() 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 string. 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.

When a partial match is returned, the first two elements in the ovector point to the portion of the subject that was matched, but the values in the rest of the ovector are undefined. The appearance of \K in the pattern has no effect for a partial match. Consider this pattern:

/abc\K123/

If it is matched against "456abc123xyz" the result is a complete match, and the ovector defines the matched string as "123", because \K resets the "start of match" point. However, if a partial match is requested and the subject string is "456abc12", a partial match is found for the string "abc12", because all these characters are needed for a subsequent re-match with additional characters.

What happens when a partial match is identified depends on which of the two partial matching options are set.

PARTIAL MATCHING USING pcre2_dfa_match()¶

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.

When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if there have been no complete matches. Otherwise, the complete matches are returned. However, if PCRE2_PARTIAL_HARD is set, a partial match takes precedence over any complete matches. The portion of the string that was matched when the longest partial match was found is set as the first matching string.

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 PCRE2_PARTIAL_HARD is set. Consider the string "dog" matched against the ungreedy pattern shown above:

/dog(sbody)??/

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 so returns that when PCRE2_PARTIAL_HARD is set.

PARTIAL MATCHING AND WORD BOUNDARIES¶

If a pattern ends with one of sequences \b or \B, which test for word boundaries, partial matching with PCRE2_PARTIAL_SOFT can give counter-intuitive results. Consider this pattern:

/\bcat\b/

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 not PCRE2_ERROR_PARTIAL. Using PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because then the partial match takes precedence.

EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST¶

If the partial_soft (or ps) modifier is present on a pcre2test data line, the PCRE2_PARTIAL_SOFT option is used for the match. Here is a run of pcre2test that uses the date example quoted above:

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> 25dec3\=ps
Partial match: 23dec3
data> 3ju\=ps
Partial match: 3ju
data> 3juj\=ps
No match
data> j\=ps
No match

The first data string is matched completely, so pcre2test 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.

If the partial_hard (or ph) modifier is present on a pcre2test data line, the PCRE2_PARTIAL_HARD option is set for the match.

MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()¶

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 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. Here is an example using pcre2test:

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

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; PCRE2 does not retain the previously partially-matched string. It is up to the calling program to do that if it needs to.

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.

You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_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.

MULTI-SEGMENT MATCHING WITH pcre2_match()¶

Unlike the DFA function, it is not possible to restart the previous match with a new segment of data when using pcre2_match(). 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.

It is best to use PCRE2_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:

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

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 function, 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.

ISSUES WITH MULTI-SEGMENT MATCHING¶

Certain types of pattern may give problems with multi-segment matching, whichever matching function is used.

1. If the pattern contains a test for the beginning of a line, you need to pass the PCRE2_NOTBOL option when the subject string for any call does start at the beginning of a line. There is also a PCRE2_NOTEOL option, but in practice when doing multi-segment matching you should be using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL.

2. If a pattern contains a lookbehind assertion, characters that precede the start of the partial match may have been inspected during the matching process. When using pcre2_match(), sufficient characters must be retained for the next match attempt. You can ensure that enough characters are retained by doing the following:

Before doing any matching, find the length of the longest lookbehind in the pattern by calling pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting count is in characters, not code units. After a partial match, moving back from the ovector[0] offset in the subject by the number of characters given for the maximum lookbehind gets you to the earliest character that must be retained. In a non-UTF or a 32-bit situation, moving back is just a subtraction, but in UTF-8 or UTF-16 you have to count characters while moving back through the code units.

Characters before the point you have now reached can be discarded, and after the next segment has been added to what is retained, you should run the next match with the startoffset argument set so that the match begins at the same point as before.

For example, if the pattern "(?<=123)abc" is partially matched against the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maximum lookbehind count is 3, so all characters before offset 2 can be discarded. The value of startoffset for the next match should be 3. When pcre2test displays a partial match, it indicates the lookbehind characters with '<' characters:

re> "(?<=123)abc"
data> xx123ab\=ph
Partial match: 123ab
<<<

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:

re> /c(?<=abc)x/
data> ab\=ps
No match

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.

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 PCRE2_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 PCRE2_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 this pcre2test example:

re> /dog(sbody)?/
data> dogsb\=ps
0: dog
data> do\=ps,dfa
Partial match: do
data> gsb\=ps,dfa,dfa_restart
0: g
data> dogsbody\=dfa
0: dogsbody
1: dog

The first data line passes the string "dogsb" to a standard matching function, setting the PCRE2_PARTIAL_SOFT option. Although the string is a partial match for "dogsbody", the result is not PCRE2_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.

Because of these problems, it is best to use PCRE2_PARTIAL_HARD when matching multi-segment data. The example above then behaves differently:

re> /dog(sbody)?/
data> dogsb\=ph
Partial match: dogsb
data> do\=ps,dfa
Partial match: do
data> gsb\=ph,dfa,dfa_restart
Partial match: gsb

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 PCRE2_DFA_RESTART is used. For example, consider this pattern:

1234|3789

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:

1234|ABCD

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:

re> /1234|3789/
data> ABC123\=ph
Partial match: 123
data> 1237890
0: 3789

Of course, instead of using PCRE2_DFA_RESTART, the same technique of re-running the entire match can also be used with the DFA matching function. Another possibility is to work with two buffers. If a partial match at offset n in the first buffer is followed by "no match" when PCRE2_DFA_RESTART is used on the second buffer, you can then try a new match starting at offset n+1 in the first buffer.

AUTHOR¶

Philip Hazel
University Computing Service
Cambridge, England.

REVISION¶

Last updated: 22 December 2014
Copyright (c) 1997-2014 University of Cambridge.