为什么正则表达式/ [\ w \ W] + x / i运行起来会非常慢？

尝试：

time perl -E '$x="a" x 100000; $x =~ /[\w\W]+x/i'  

将运行很长时间（在我的笔记本上20秒）。没有/i，例如

time perl -E '$x="a" x 100000; $x =~ /[\w\W]+x/' 

在0.07秒内完成。

不管正则表达式[\w\W]没有多大意义，这种巨大的差异使我感到惊讶。

为什么会有如此大的差异？

编辑

更准确地说：

$ time perl -E '$x="a" x 100000; $x =~ /[\w\W]+x/i'  

real    0m19.479s
user    0m19.419s
sys 0m0.038s

我的 perl

Summary of my perl5 (revision 5 version 20 subversion 3) configuration:

  Platform:
    osname=darwin, osvers=15.0.0, archname=darwin-2level
    uname='darwin nox.local 15.0.0 darwin kernel version 15.0.0: sat sep 19 15:53:46 pdt 2015; root:xnu-3247.10.11~1release_x86_64 x86_64 '
    config_args='-Dprefix=/opt/anyenv/envs/plenv/versions/5.20.3 -de -Dusedevel -A'eval:scriptdir=/opt/anyenv/envs/plenv/versions/5.20.3/bin''
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    use64bitint=define, use64bitall=define, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fno-common -DPERL_DARWIN -fno-strict-aliasing -pipe -fstack-protector -I/opt/local/include',
    optimize='-O3',
    cppflags='-fno-common -DPERL_DARWIN -fno-strict-aliasing -pipe -fstack-protector -I/opt/local/include'
    ccversion='', gccversion='4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.1.76)', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags =' -fstack-protector -L/opt/local/lib'
    libpth=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/7.0.0/lib /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib /usr/lib /opt/local/lib
    libs=-lpthread -lgdbm -ldbm -ldl -lm -lutil -lc
    perllibs=-lpthread -ldl -lm -lutil -lc
    libc=, so=dylib, useshrplib=false, libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags=' -bundle -undefined dynamic_lookup -L/opt/local/lib -fstack-protector'


Characteristics of this binary (from libperl): 
  Compile-time options: HAS_TIMES PERLIO_LAYERS PERL_DONT_CREATE_GVSV
                        PERL_HASH_FUNC_ONE_AT_A_TIME_HARD PERL_MALLOC_WRAP
                        PERL_NEW_COPY_ON_WRITE PERL_PRESERVE_IVUV
                        PERL_USE_DEVEL USE_64_BIT_ALL USE_64_BIT_INT
                        USE_LARGE_FILES USE_LOCALE USE_LOCALE_COLLATE
                        USE_LOCALE_CTYPE USE_LOCALE_NUMERIC USE_PERLIO
                        USE_PERL_ATOF
  Locally applied patches:
    Devel::PatchPerl 1.38
  Built under darwin
  Compiled at Oct 28 2015 14:46:19
  @INC:
    /opt/anyenv/envs/plenv/versions/5.20.3/lib/perl5/site_perl/5.20.3/darwin-2level
    /opt/anyenv/envs/plenv/versions/5.20.3/lib/perl5/site_perl/5.20.3
    /opt/anyenv/envs/plenv/versions/5.20.3/lib/perl5/5.20.3/darwin-2level
    /opt/anyenv/envs/plenv/versions/5.20.3/lib/perl5/5.20.3
    .

从问题的背景来看：实际代码将字符串与大量正则表达式（某种反垃圾邮件）匹配，因此我无法可靠地手动检查正则表达式数据库。真正的代码片段是

sub docheck {
    ...
    ...
    foreach my $regex (@$regexs) {
        if ( $_[0] =~ /$regex/i ) {

并且[\w\W]+是10k regexes之一:(，例如：[\w\W]+medicine\.netfirms\.com-regex-DB可能需要清理-但是...您知道:)

现在，代码已更改：

sub docheck {
    ...
    my $str = lc($_[0]);
    foreach my $regex (@$regexs) {
        if ( $str =~ /$regex/ ) {

所以避免/i。