我一直认为你不能在零宽度断言中使用重复量词(Perl Compatible Regular Expressions [PCRE]).但是,最近我发现你可以在前瞻断言中使用它们.
所以我的问题是:
PCRE正则表达式引擎在使用零宽度外观搜索时如何工作,从而无法使用重复量词？
以下是R中PCRE的一个简单示例:
# Our string
x <- 'MaaabcccM'
## Does it contain a 'b', preceeded by an 'a' and followed by zero or more 'c',
## then an 'M'?
grepl( '(?<=a)b(?=c*M)' , x , perl=T )
# [1] TRUE
## Does it contain a 'b': (1) preceeded by an 'M' and then zero or more 'a' and
## (2) followed by zero or more 'c' then an 'M'?
grepl( '(?<=Ma*)b(?=c*M)' , x , perl = TRUE )
# Error in grepl("(?<=Ma*)b(?=c*M)", x, perl = TRUE) :
# invalid regular expression '(?<M=a*)b(?=c*M)'
# In addition: Warning message:
# In grepl("(?<=Ma*)b(?=c*M)", x, perl = TRUE) : PCRE pattern compilation error
# 'lookbehind assertion is not fixed length'
# at ')b(?=c*M)'
zx8*_*x81 35
这个问题的最终答案是在引擎的代码中,在答案的最底部,您将能够深入到PCRE引擎代码的部分,该代码负责确保外观中的固定长度 - 如果您对此感兴趣了解最精细的细节.与此同时,让我们逐步放大更高层次的问题.
可变宽度Lookbehind与无限宽度Lookbehind
首先,快速澄清条款.越来越多的引擎(包括PCRE)支持某种形式的可变宽度后视,其中变化落在确定的范围内,例如:
相比之下,在无限宽度的后视中,您可以使用量化的标记,例如 a+
支持无限宽度外观的引擎
为了记录,这些引擎支持无限的lookbehind:
regexPython模块据我所知,他们是唯一的.
PCRE中的可变后视
在PCRE中,文档中最相关的部分是:
lookbehind断言的内容受到限制,因此它匹配的所有字符串必须具有固定长度.但是,如果有几个顶级替代品,它们并非都必须具有相同的固定长度.
因此,以下lookbehind是有效的:
(?<=a |big )cat
但是,这些都不是:
(?<=a\s?|big )cat (交替的两侧没有固定的宽度)(?<=@{1,10})cat (可变宽度)(?<=\R)cat(\R不具有固定宽度的,因为它可以匹配\n,\r\n等)(?<=\X)cat(\X没有固定宽度,因为Unicode字形集群可以包含可变数量的字节.)(?<=a+)cat (显然没有修复)具有零宽度匹配但无限重复的Lookbehind
现在考虑一下:
(?<=(?=@+))(cat#+)
从表面上看,这是一个固定宽度的后视,因为它只能找到零宽度匹配(由前瞻定义(?=@++)).这是解决无限外观限制的伎俩吗？
不会.PCRE会扼杀这个.即使后视镜的内容为零宽度,PCRE也不允许在后视镜中进行无限重复.无处不在.当文档说它匹配的所有字符串必须有一个固定长度时,它应该是:
任何组件匹配的所有字符串都必须具有固定长度.
解决方法:没有无限外观的生活
在PCRE中,无限外观有助于解决问题的两个主要解决方案是\K捕获组.
解决方法#1: \K
该\K断言告诉引擎下降从它返回的最后一场比赛,到目前为止什么匹配.
假设你想要(?<=@+)cat#+,这在PCRE中是不合法的.相反,你可以使用:
@+\Kcat#+
解决方法#2:捕获组
另一种继续进行的方法是匹配您在后视镜中放置的任何内容,并捕获捕获组中感兴趣的内容.然后,您从捕获组中检索匹配项.
例如(?<=@+)cat#+,您将使用以下代码:
@+(cat#+)
在R中,这可能如下所示:
matches <- regexpr("@+(cat#+)", subject, perl=TRUE);
result <- attr(matches, "capture.start")[,1]
attr(result, "match.length") <- attr(matches, "capture.length")[,1]
regmatches(subject, result)
在不支持的语言中\K,这通常是唯一的解决方案.
引擎内部:PCRE代码说什么？
最终的答案是找到pcre_compile.c.如果您检查以此注释开头的代码块:
如果是lookbehind,请检查此分支是否与固定长度的字符串匹配
你发现咕噜咕噜的工作是由find_fixedlength()函数完成的.
我在这里为任何想要深入了解更多细节的人重现它.
static int
find_fixedlength(pcre_uchar *code, BOOL utf, BOOL atend, compile_data *cd)
{
int length = -1;
register int branchlength = 0;
register pcre_uchar *cc = code + 1 + LINK_SIZE;
/* Scan along the opcodes for this branch. If we get to the end of the
branch, check the length against that of the other branches. */
for (;;)
{
int d;
pcre_uchar *ce, *cs;
register pcre_uchar op = *cc;
switch (op)
{
/* We only need to continue for OP_CBRA (normal capturing bracket) and
OP_BRA (normal non-capturing bracket) because the other variants of these
opcodes are all concerned with unlimited repeated groups, which of course
are not of fixed length. */
case OP_CBRA:
case OP_BRA:
case OP_ONCE:
case OP_ONCE_NC:
case OP_COND:
d = find_fixedlength(cc + ((op == OP_CBRA)? IMM2_SIZE : 0), utf, atend, cd);
if (d < 0) return d;
branchlength += d;
do cc += GET(cc, 1); while (*cc == OP_ALT);
cc += 1 + LINK_SIZE;
break;
/* Reached end of a branch; if it's a ket it is the end of a nested call.
If it's ALT it is an alternation in a nested call. An ACCEPT is effectively
an ALT. If it is END it's the end of the outer call. All can be handled by
the same code. Note that we must not include the OP_KETRxxx opcodes here,
because they all imply an unlimited repeat. */
case OP_ALT:
case OP_KET:
case OP_END:
case OP_ACCEPT:
case OP_ASSERT_ACCEPT:
if (length < 0) length = branchlength;
else if (length != branchlength) return -1;
if (*cc != OP_ALT) return length;
cc += 1 + LINK_SIZE;
branchlength = 0;
break;
/* A true recursion implies not fixed length, but a subroutine call may
be OK. If the subroutine is a forward reference, we can't deal with
it until the end of the pattern, so return -3. */
case OP_RECURSE:
if (!atend) return -3;
cs = ce = (pcre_uchar *)cd->start_code + GET(cc, 1); /* Start subpattern */
do ce += GET(ce, 1); while (*ce == OP_ALT); /* End subpattern */
if (cc > cs && cc < ce) return -1; /* Recursion */
d = find_fixedlength(cs + IMM2_SIZE, utf, atend, cd);
if (d < 0) return d;
branchlength += d;
cc += 1 + LINK_SIZE;
break;
/* Skip over assertive subpatterns */
case OP_ASSERT:
case OP_ASSERT_NOT:
case OP_ASSERTBACK:
case OP_ASSERTBACK_NOT:
do cc += GET(cc, 1); while (*cc == OP_ALT);
cc += PRIV(OP_lengths)[*cc];
break;
/* Skip over things that don't match chars */
case OP_MARK:
case OP_PRUNE_ARG:
case OP_SKIP_ARG:
case OP_THEN_ARG:
cc += cc[1] + PRIV(OP_lengths)[*cc];
break;
case OP_CALLOUT:
case OP_CIRC:
case OP_CIRCM:
case OP_CLOSE:
case OP_COMMIT:
case OP_CREF:
case OP_DEF:
case OP_DNCREF:
case OP_DNRREF:
case OP_DOLL:
case OP_DOLLM:
case OP_EOD:
case OP_EODN:
case OP_FAIL:
case OP_NOT_WORD_BOUNDARY:
case OP_PRUNE:
case OP_REVERSE:
case OP_RREF:
case OP_SET_SOM:
case OP_SKIP:
case OP_SOD:
case OP_SOM:
case OP_THEN:
case OP_WORD_BOUNDARY:
cc += PRIV(OP_lengths)[*cc];
break;
/* Handle literal characters */
case OP_CHAR:
case OP_CHARI:
case OP_NOT:
case OP_NOTI:
branchlength++;
cc += 2;
#ifdef SUPPORT_UTF
if (utf && HAS_EXTRALEN(cc[-1])) cc += GET_EXTRALEN(cc[-1]);
#endif
break;
/* Handle exact repetitions. The count is already in characters, but we
need to skip over a multibyte character in UTF8 mode. */
case OP_EXACT:
case OP_EXACTI:
case OP_NOTEXACT:
case OP_NOTEXACTI:
branchlength += (int)GET2(cc,1);
cc += 2 + IMM2_SIZE;
#ifdef SUPPORT_UTF
if (utf && HAS_EXTRALEN(cc[-1])) cc += GET_EXTRALEN(cc[-1]);
#endif
break;
case OP_TYPEEXACT:
branchlength += GET2(cc,1);
if (cc[1 + IMM2_SIZE] == OP_PROP || cc[1 + IMM2_SIZE] == OP_NOTPROP)
cc += 2;
cc += 1 + IMM2_SIZE + 1;
break;
/* Handle single-char matchers */
case OP_PROP:
case OP_NOTPROP:
cc += 2;
/* Fall through */
case OP_HSPACE:
case OP_VSPACE:
case OP_NOT_HSPACE:
case OP_NOT_VSPACE:
case OP_NOT_DIGIT:
case OP_DIGIT:
case OP_NOT_WHITESPACE:
case OP_WHITESPACE:
case OP_NOT_WORDCHAR:
case OP_WORDCHAR:
case OP_ANY:
case OP_ALLANY:
branchlength++;
cc++;
break;
/* The single-byte matcher isn't allowed. This only happens in UTF-8 mode;
otherwise \C is coded as OP_ALLANY. */
case OP_ANYBYTE:
return -2;
/* Check a class for variable quantification */
case OP_CLASS:
case OP_NCLASS:
#if defined SUPPORT_UTF || defined COMPILE_PCRE16 || defined COMPILE_PCRE32
case OP_XCLASS:
/* The original code caused an unsigned overflow in 64 bit systems,
so now we use a conditional statement. */
if (op == OP_XCLASS)
cc += GET(cc, 1);
else
cc += PRIV(OP_lengths)[OP_CLASS];
#else
cc += PRIV(OP_lengths)[OP_CLASS];
#endif
switch (*cc)
{
case OP_CRSTAR:
case OP_CRMINSTAR:
case OP_CRPLUS:
case OP_CRMINPLUS:
case OP_CRQUERY:
case OP_CRMINQUERY:
case OP_CRPOSSTAR:
case OP_CRPOSPLUS:
case OP_CRPOSQUERY:
return -1;
case OP_CRRANGE:
case OP_CRMINRANGE:
case OP_CRPOSRANGE:
if (GET2(cc,1) != GET2(cc,1+IMM2_SIZE)) return -1;
branchlength += (int)GET2(cc,1);
cc += 1 + 2 * IMM2_SIZE;
break;
default:
branchlength++;
}
break;
/* Anything else is variable length */
case OP_ANYNL:
case OP_BRAMINZERO:
case OP_BRAPOS:
case OP_BRAPOSZERO:
case OP_BRAZERO:
case OP_CBRAPOS:
case OP_EXTUNI:
case OP_KETRMAX:
case OP_KETRMIN:
case OP_KETRPOS:
case OP_MINPLUS:
case OP_MINPLUSI:
case OP_MINQUERY:
case OP_MINQUERYI:
case OP_MINSTAR:
case OP_MINSTARI:
case OP_MINUPTO:
case OP_MINUPTOI:
case OP_NOTMINPLUS:
case OP_NOTMINPLUSI:
case OP_NOTMINQUERY:
case OP_NOTMINQUERYI:
case OP_NOTMINSTAR:
case OP_NOTMINSTARI:
case OP_NOTMINUPTO:
case OP_NOTMINUPTOI:
case OP_NOTPLUS:
case OP_NOTPLUSI:
case OP_NOTPOSPLUS:
case OP_NOTPOSPLUSI:
case OP_NOTPOSQUERY:
case OP_NOTPOSQUERYI:
case OP_NOTPOSSTAR:
case OP_NOTPOSSTARI:
case OP_NOTPOSUPTO:
case OP_NOTPOSUPTOI:
case OP_NOTQUERY:
case OP_NOTQUERYI:
case OP_NOTSTAR:
case OP_NOTSTARI:
case OP_NOTUPTO:
case OP_NOTUPTOI:
case OP_PLUS:
case OP_PLUSI:
case OP_POSPLUS:
case OP_POSPLUSI:
case OP_POSQUERY:
case OP_POSQUERYI:
case OP_POSSTAR:
case OP_POSSTARI:
case OP_POSUPTO:
case OP_POSUPTOI:
case OP_QUERY:
case OP_QUERYI:
case OP_REF:
case OP_REFI:
case OP_DNREF:
case OP_DNREFI:
case OP_SBRA:
case OP_SBRAPOS:
case OP_SCBRA:
case OP_SCBRAPOS:
case OP_SCOND:
case OP_SKIPZERO:
case OP_STAR:
case OP_STARI:
case OP_TYPEMINPLUS:
case OP_TYPEMINQUERY:
case OP_TYPEMINSTAR:
case OP_TYPEMINUPTO:
case OP_TYPEPLUS:
case OP_TYPEPOSPLUS:
case OP_TYPEPOSQUERY:
case OP_TYPEPOSSTAR:
case OP_TYPEPOSUPTO:
case OP_TYPEQUERY:
case OP_TYPESTAR:
case OP_TYPEUPTO:
case OP_UPTO:
case OP_UPTOI:
return -1;
/* Catch unrecognized opcodes so that when new ones are added they
are not forgotten, as has happened in the past. */
default:
return -4;
}
}
/* Control never gets here */
}
正则表达式引擎设计为从左到右工作.
对于前瞻,引擎匹配当前位置右侧的整个文本.但是,对于lookbehinds,正则表达式引擎确定要退回的字符串长度,然后检查匹配(再次从左到右).
所以,如果你提供一些无限的量词像*或者+,回顾后不会工作,因为发动机不知道要走多少步向后走.
我将举例说明lookbehind是如何工作的(尽管这个例子非常愚蠢).
假设您要匹配姓氏Panta,只有当第一个名称长度为5-7个字符时.
我们来取字符串:
Full name is Subigya Panta.
考虑正则表达式:
(?<=\b\w{5,7}\b)\sPanta
引擎确认存在正向后观,因此它首先搜索该单词Panta(在它之前带有空白字符).这是一场比赛.
现在,引擎看起来与lookbehind内的正则表达式相匹配.它向后退7个字符(因为量词是贪婪的).单词边界匹配空间和空间之间的位置S.然后它匹配所有7个字符,然后下一个字边界匹配a空间之间的位置.
lookbehind内的正则表达式是匹配的,因此整个正则表达式返回true,因为匹配的字符串包含Panta.(注意,环绕声断言是零宽度,不消耗任何字符.)