PCRE2PARTIAL(3)

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💖 PCRE2PARTIAL(3) — the partial matching page, bestie edition 💖

Library Functions Manual · PCRE2PARTIAL(3)

✨ NAME ✨

PCRE2 — Perl-compatible regular expressions (revised API), and specifically the partial matching of them 🦋

🌸 PARTIAL MATCHING IN PCRE2 🌸

okay so picture this: normal PCRE2 life, right? If there's a match all the way up to the end of your subject string, but you'd need more characters to match the entire pattern, PCRE2 just hits you with PCRE2_ERROR_NOMATCH — same as any other failed match, no drama, no explanation. But lowkey there are situations where you really really want to tell that "partial match" case apart from a plain old nope. 👑

One example, bestie: an app where the subject string is super long and you don't have all of it at once. What you wanna do is match segment by segment, but you need special handling for when a matched substring spans the boundary between two segments. 🧚

Another example: checking a user's input string as they type it, making sure it conforms to a required format. Invalid characters can be diagnosed and rejected immediately — instant feedback, no cap. 💅

So here's the tea: partial matching is a PCRE2-specific feature — it is not Perl-compatible, sorry to the Perl girlies. You ask for it by setting one of the PCRE2_PARTIAL_HARD or PCRE2_PARTIAL_SOFT options when you call a matching function. The difference between the two? Whether a partial match is preferred over an alternative complete match — though the deets differ between the two types of matching function. And if you set both options, PCRE2_PARTIAL_HARD takes precedence. Periodt. 💀

Now, if you want partial matching with just-in-time optimized code, you can't just set a partial match option on the matching function — you also have to call pcre2_jit_compile() with one or both of these:

PCRE2_JIT_PARTIAL_HARD
PCRE2_JIT_PARTIAL_SOFT

You should also set PCRE2_JIT_COMPLETE if you're gonna run non-partial matches on the same pattern. Separate code gets compiled for each mode. And if the appropriate JIT mode hasn't been compiled, interpretive matching code is used instead. 🫶

Setting a partial matching option disables two of PCRE2's standard optimization hints. Let me explain:

PCRE2 remembers the last literal code unit in a pattern, and bails out of matching immediately if it isn't present in the subject string. This optimization cannot be used for a subject string that might match only partially.
PCRE2 also remembers a minimum length of a matching string, and doesn't bother running the matching function on shorter strings. This optimization is also disabled for partial matching.

🌈 REQUIREMENTS FOR A PARTIAL MATCH 🌈

A possible partial match happens during matching when the end of the subject string is reached successfully, but either more characters are needed to complete the match, or adding more characters might change what is matched. 👀

Example 1: if the pattern is /abc/ and the subject is "ab", more characters are definitely needed to complete a match. In this case both hard and soft matching options yield a partial match. ✨

Example 2: if the pattern is /ab+/ and the subject is "ab", a complete match can be found, but adding more characters might change what is matched. In this case, only PCRE2_PARTIAL_HARD returns a partial match; PCRE2_PARTIAL_SOFT returns the complete match. 🙈

So on reaching the end of the subject, when PCRE2_PARTIAL_HARD is set, if the next pattern 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 a character, and at least one of the following must be true:

At least one character has already been inspected. An inspected 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 pattern contains one or more lookbehind assertions. This condition exists in case there's a lookbehind that inspects characters before the start of the match.
There's 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 is a possibility, and if PCRE2_PARTIAL_SOFT is set and neither of the above conditions is true, it is returned. However — and this is the important bit — because adding more characters might 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 until some more characters are added, we do not know if it will be an empty string or something longer". 💅

🎀 PARTIAL MATCHING USING `pcre2_match()` 🎀

When a partial matching option is set, the result of calling pcre2_match() can be one of the following:

A successful match — a complete match has been found, starting and ending within this subject. ✨
PCRE2_ERROR_NOMATCH — no match can start anywhere in this subject. 💀
PCRE2_ERROR_PARTIAL — adding more characters may result in a complete match that uses one or more characters from the end of this 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. And heads up: the appearance of \K in the pattern has no effect for a partial match. Consider this pattern:

/abc\K123/

If it's 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. BUT — 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. 👑

If there's more than one partial match, the first one that was found provides the data that is returned. Consider this pattern:

/123\w+X|dogY/

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 PCRE2_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 partial matches, because "dog" on its own partially matches the second alternative.) 🐶

🦋 How a partial match is processed by `pcre2_match()` 🦋

What happens when a partial match is identified depends on which of the two partial matching options is set. Let's get into it.

If PCRE2_PARTIAL_HARD is set, PCRE2_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 is not the true end of the available data — which is exactly why \z, \Z, \b, \B, and $ always give a partial match. 💅

If PCRE2_PARTIAL_SOFT is set, 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, PCRE2_ERROR_PARTIAL is returned instead of PCRE2_ERROR_NOMATCH. 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; \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. 🌸

The difference between the two partial matching options can be illustrated by a pattern such as:

/dog(sbody)?/

This matches either "dog" or "dogsbody", greedily (that is, it prefers the longer string if possible). If it's matched against the string "dog" with PCRE2_PARTIAL_SOFT, it yields a complete match for "dog". However, if PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PARTIAL. 🐕

On the other hand, if the pattern is made ungreedy the result is different:

/dog(sbody)??/

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:

/dog(sbody)?/    is the same as  /dogsbody|dog/
/dog(sbody)??/   is the same as  /dog|dogsbody/

The second pattern will never match "dogsbody", because it will always find the shorter match first. 😭

🍓 Example of partial matching using `pcre2test` 🍓

The pcre2test data modifiers partial_hard (or ph) and partial_soft (or ps) set PCRE2_PARTIAL_HARD and PCRE2_PARTIAL_SOFT, respectively, when calling pcre2_match(). Here's a run of pcre2test using a pattern that matches the whole subject in the form of a date:

    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

This example gives the same results for both hard and soft partial matching options. But here's an example where there is a difference:

    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

With PCRE2_PARTIAL_SOFT, the subject is matched completely. For PCRE2_PARTIAL_HARD, however, the subject is assumed not to be complete, so there's only a partial match. 💅

this page is long so it's split in two — part 2 is coming, don't go anywhere bestie 🫶

🌈 MULTI-SEGMENT MATCHING WITH pcre2_match() 🌈

Real talk: PCRE was not originally designed with multi-segment matching in mind. 💀 But over time, features (including partial matching) that make multi-segment matching possible have been added. So now you can search a very long string segment by segment by calling pcre2_match() repeatedly, with the aim of achieving the same results that would happen if the entire string was available for searching all the time. Normally, the strings that are being sought are much shorter than each individual segment, and are in the middle of very long strings, so the pattern is normally not anchored. ✨

Special logic must be implemented to handle a matched substring that spans a segment boundary. PCRE2_PARTIAL_HARD should be used, because it returns a partial match at the end of a segment whenever there is the possibility of changing the match by adding more characters. The PCRE2_NOTBOL option should also be set for all but the first segment. 👑

When a partial match occurs, the next segment must be added to the current subject and the match re-run, using the startoffset argument of pcre2_match() to begin at the point where the partial match started. For example:

    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

Note the use of the offset modifier to start the new match where the partial match was found. In this example, the next segment was added to the one in which the partial match was found. This is the most straightforward approach, typically using a memory buffer that is twice the size of each segment. After a partial match, the first half of the buffer is discarded, the second half is moved to the start of the buffer, and a new segment is added before repeating the match as in the example above. After a no match, the entire buffer can be discarded. 🗑️

If there are memory constraints, you may want to discard text that precedes a partial match before adding the next segment. Unfortunately, this is not at present straightforward. 😭 In cases such as the above, where the pattern does not contain any lookbehinds, it is sufficient to retain only the partially matched substring. However, if the pattern contains a lookbehind assertion, characters that precede the start of the partial match may have been inspected during the matching process. When pcre2test displays a partial match, it indicates these characters with '<' if the allusedtext modifier is set:

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

However, the allusedtext 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 therefore not possible in general to obtain the exact number of characters that must be retained in order to get the right match result. If you cannot retain the entire segment, you must find some heuristic way of choosing. 🤷‍♀️

If you know the approximate length of the matching substrings, you can use that to decide how much text to retain. The only lookbehind information that is currently available via the API is the length of the longest individual lookbehind in a pattern, but this can be misleading if there are nested lookbehinds. The value returned by calling pcre2_pattern_info() with the PCRE2_INFO_MAXLOOKBEHIND option is the maximum number of characters (not code units) that any individual lookbehind moves back when it is processed. A pattern such as "(?<=(?<!b)a)" has a maximum lookbehind value of one, but inspects two characters before its starting point. 😳

In a non-UTF or a 32-bit case, 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. 💅

🦋 PARTIAL MATCHING USING pcre2_dfa_match() 🦋

The DFA function moves 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. 👀

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. 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 function always searches for all possible matches, and there is no difference between greedy and ungreedy repetition, its behaviour is different from the pcre2_match(). Consider the string "dog" matched against this ungreedy pattern:

  /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. 💖

🧚 MULTI-SEGMENT MATCHING WITH pcre2_dfa_match() 🧚

When a partial match has been found using the 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. You can set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with PCRE2_DFA_RESTART to continue partial matching over multiple segments. 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. This 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. 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. Depending on the application, this may or may not be what you want. 🤔

If you do want to allow for starting again at the next character, one way of doing it is to retain some or all of the segment and try a new complete match, as described for pcre2_match() above. 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. 🧠

Therefore, PCRE2_DFA_RESTART does not remove the need to keep earlier subject data. In order to perform a bumpalong and continue matching at the next character, all subject data from the current match start must be retained, together with enough preceding data to satisfy the requirements of lookbehind assertions (remembering as above that PCRE2_INFO_MAXLOOKBEHIND only describes the buffer requirement of a single lookbehind, and nested lookbehinds can read back further from the match start). This makes streaming match processing using PCRE2_DFA_RESTART challenging, and the application cannot easily set a bound on the number of chunks to buffer. For multi-segment searching that is intended to produce the same result as searching the whole subject, it is usually simpler to avoid PCRE2_DFA_RESTART and run a fresh match, using either matching function, over the retained text. 💯

👑 AUTHOR 👑

Philip Hazel

Retired from University Computing Service

Cambridge, England.

🌸 REVISION 🌸

Last updated: 16 August 2026

💖 That's the tea, bestie! This was PCRE2PARTIAL(3) from PCRE2 10.49, dated 16 August 2026. Stay iconic! 💖

📦 where this page comes from

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💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-14: a language model's rewrite of the original, under its terms