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version 1.1.1.4, 2013/07/22 08:25:56
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.TH PCREPARTIAL 3 | .TH PCREPARTIAL 3 "20 February 2013" "PCRE 8.33" |
.SH NAME |
.SH NAME |
PCRE - Perl-compatible regular expressions |
PCRE - Perl-compatible regular expressions |
.SH "PARTIAL MATCHING IN PCRE" |
.SH "PARTIAL MATCHING IN PCRE" |
Line 32 or not a partial match is preferred to an alternative
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Line 32 or not a partial match is preferred to an alternative
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the details differ between the two types of matching function. If both options |
the details differ between the two types of matching function. If both options |
are set, PCRE_PARTIAL_HARD takes precedence. |
are set, PCRE_PARTIAL_HARD takes precedence. |
.P |
.P |
Setting a partial matching option disables the use of any just-in-time code | If you want to use partial matching with just-in-time optimized code, you must |
that was set up by studying the compiled pattern with the | call \fBpcre_study()\fP, \fBpcre16_study()\fP or \fBpcre32_study()\fP with one |
PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard | 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 |
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 |
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 |
optimization cannot be used for a subject string that might match only |
Line 43 matching string, and does not bother to run the matchi
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Line 52 matching string, and does not bother to run the matchi
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strings. This optimization is also disabled for partial matching. |
strings. This optimization is also disabled for partial matching. |
. |
. |
. |
. |
.SH "PARTIAL MATCHING USING pcre_exec() OR pcre16_exec()" | .SH "PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec()" |
.rs |
.rs |
.sp |
.sp |
A partial match occurs during a call to \fBpcre_exec()\fP or |
A partial match occurs during a call to \fBpcre_exec()\fP or |
\fBpcre16_exec()\fP when the end of the subject string is reached successfully, | \fBpcre[16|32]_exec()\fP when the end of the subject string is reached |
but matching cannot continue because more characters are needed. However, at | successfully, but matching cannot continue because more characters are needed. |
least one character in the subject must have been inspected. This character | However, at least one character in the subject must have been inspected. This |
need not form part of the final matched string; lookbehind assertions and the | character need not form part of the final matched string; lookbehind assertions |
\eK escape sequence provide ways of inspecting characters before the start of a | and the \eK escape sequence provide ways of inspecting characters before the |
matched substring. The requirement for inspecting at least one character exists | start of a matched substring. The requirement for inspecting at least one |
because an empty string can always be matched; without such a restriction there | character exists because an empty string can always be matched; without such a |
would always be a partial match of an empty string at the end of the subject. | 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 |
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 |
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 |
was inspected. For convenience, the second offset points to the end of the |
subject so that a substring can easily be identified. | 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 first offset identifies the start of the | For the majority of patterns, the contents of the first and third slots will be |
partially matched string. However, for patterns that contain lookbehind | the same. However, for patterns that contain lookbehind assertions, or begin |
assertions, or \eK, or begin with \eb or \eB, earlier characters have been | with \eb or \eB, characters before the one where matching started may have been |
inspected while carrying out the match. For example: | inspected while carrying out the match. For example, consider this pattern: |
.sp |
.sp |
/(?<=abc)123/ |
/(?<=abc)123/ |
.sp |
.sp |
This pattern matches "123", but only if it is preceded by "abc". If the subject |
This pattern matches "123", but only if it is preceded by "abc". If the subject |
string is "xyzabc12", the offsets after a partial match are for the substring | string is "xyzabc12", the first two offsets after a partial match are for the |
"abc12", because all these characters are needed if another match is tried | substring "abc12", because all these characters were inspected. However, the |
with extra characters added to the subject. | 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 |
What happens when a partial match is identified depends on which of the two |
partial matching options are set. |
partial matching options are set. |
. |
. |
. |
. |
.SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre16_exec()" | .SS "PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec()" |
.rs |
.rs |
.sp |
.sp |
If PCRE_PARTIAL_SOFT is set when \fBpcre_exec()\fP or \fBpcre16_exec()\fP | 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 |
identifies a partial match, the partial match is remembered, but matching |
continues as normal, and other alternatives in the pattern are tried. If no |
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 |
complete match can be found, PCRE_ERROR_PARTIAL is returned instead of |
Line 105 example, there are two partial matches, because "dog"
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Line 117 example, there are two partial matches, because "dog"
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matches the second alternative.) |
matches the second alternative.) |
. |
. |
. |
. |
.SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre16_exec()" | .SS "PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec()" |
.rs |
.rs |
.sp |
.sp |
If PCRE_PARTIAL_HARD is set for \fBpcre_exec()\fP or \fBpcre16_exec()\fP, | 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 |
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" |
continuing to search for possible complete matches. This option is "hard" |
because it prefers an earlier partial match over a later complete match. For |
because it prefers an earlier partial match over a later complete match. For |
Line 153 The second pattern will never match "dogsbody", becaus
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Line 165 The second pattern will never match "dogsbody", becaus
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shorter match first. |
shorter match first. |
. |
. |
. |
. |
.SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre16_dfa_exec()" | .SH "PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec()" |
.rs |
.rs |
.sp |
.sp |
The DFA functions move along the subject string character by character, without |
The DFA functions move along the subject string character by character, without |
Line 245 If the escape sequence \eP is present more than once i
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Line 257 If the escape sequence \eP is present more than once i
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line, the PCRE_PARTIAL_HARD option is set for the match. |
line, the PCRE_PARTIAL_HARD option is set for the match. |
. |
. |
. |
. |
.SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre16_dfa_exec()" | .SH "MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec()" |
.rs |
.rs |
.sp |
.sp |
When a partial match has been found using a DFA matching function, it is |
When a partial match has been found using a DFA matching function, it is |
Line 274 facility can be used to pass very long subject strings
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Line 286 facility can be used to pass very long subject strings
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functions. |
functions. |
. |
. |
. |
. |
.SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre16_exec()" | .SH "MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec()" |
.rs |
.rs |
.sp |
.sp |
From release 8.00, the standard matching functions can also be used to do |
From release 8.00, the standard matching functions can also be used to do |
Line 293 treat the end of a segment as the end of the subject w
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Line 305 treat the end of a segment as the end of the subject w
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.sp |
.sp |
At this stage, an application could discard the text preceding "23ja", add on |
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 |
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 | DFA matching functions, the entire matching string must always be available, |
the complete matching process occurs for each call, so more memory and more | and the complete matching process occurs for each call, so more memory and more |
processing time is needed. |
processing time is needed. |
.P |
.P |
\fBNote:\fP If the pattern contains lookbehind assertions, or \eK, or starts |
\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 |
with \eb or \eB, the string that is returned for a partial match includes |
characters that precede the partially matched string itself, because these must | characters that precede the start of what would be returned for a complete |
be retained when adding on more characters for a subsequent matching attempt. | match, because it contains all the characters that were inspected during the |
| partial match. |
. |
. |
. |
. |
.SH "ISSUES WITH MULTI-SEGMENT MATCHING" |
.SH "ISSUES WITH MULTI-SEGMENT MATCHING" |
Line 315 beginning of a line. There is also a PCRE_NOTEOL optio
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Line 328 beginning of a line. There is also a PCRE_NOTEOL optio
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doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which |
doing multi-segment matching you should be using PCRE_PARTIAL_HARD, which |
includes the effect of PCRE_NOTEOL. |
includes the effect of PCRE_NOTEOL. |
.P |
.P |
2. Lookbehind assertions at the start of a pattern are catered for in the | 2. Lookbehind assertions that have already been obeyed are catered for in the |
offsets that are returned for a partial match. However, in theory, a lookbehind | offsets that are returned for a partial match. However a lookbehind assertion |
assertion later in the pattern could require even earlier characters to be | later in the pattern could require even earlier characters to be inspected. You |
inspected, and it might not have been reached when a partial match occurs. This | can handle this case by using the PCRE_INFO_MAXLOOKBEHIND option of the |
is probably an extremely unlikely case; you could guard against it to a certain | \fBpcre_fullinfo()\fP or \fBpcre[16|32]_fullinfo()\fP functions to obtain the |
extent by always including extra characters at the start. | 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 |
3. Matching a subject string that is split into multiple segments may not | 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, |
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 |
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 |
Word Boundaries" above describes an issue that arises if the pattern ends with |
Line 363 multi-segment data. The example above then behaves dif
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Line 413 multi-segment data. The example above then behaves dif
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data> gsb\eR\eP\eP\eD |
data> gsb\eR\eP\eP\eD |
Partial match: gsb |
Partial match: gsb |
.sp |
.sp |
4. Patterns that contain alternatives at the top level which do not all start | 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 |
with the same pattern item may not work as expected when PCRE_DFA_RESTART is |
used. For example, consider this pattern: |
used. For example, consider this pattern: |
.sp |
.sp |
Line 412 Cambridge CB2 3QH, England.
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Line 462 Cambridge CB2 3QH, England.
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.rs |
.rs |
.sp |
.sp |
.nf |
.nf |
Last updated: 21 January 2012 | Last updated: 20 February 2013 |
Copyright (c) 1997-2012 University of Cambridge. | Copyright (c) 1997-2013 University of Cambridge. |
.fi |
.fi |