Python中的慢正则表达式？

Question

我正在尝试匹配这些类型的字符串

{@csm.foo.bar}

而不匹配其中的任何一个

{@csm.foo.bar-@csm.ooga.booga}
{@csm.foo.bar-42}

我使用的正则表达式是

r"\{@csm.((?:[a-zA-Z0-9_]+\.?)+)\}"

如果字符串包含多个匹配项，则会变得很慢。为什么？如果我去掉大括号，它会跑得很快，就像这样

r"@csm.((?:[a-zA-Z0-9_]+\.?)+)"

但这不是我想要的。

有什么想法吗？

以下是示例输入：

<dockLayout id="popup" y="0" x="0" width="{@csm.screenWidth}" height="{@csm.screenHeight}">
  <dataNumber id="selopacity_Volt" name="selopacity_Volt" value="0" />
  <dataNumber id="selopacity_Amp" name="selopacity_Amp" value="0" />
  <animate  trigger="{@m_ds_ML.VIMPBM_BatteryVoltage.valstr}" triggerOn="*"  targetNode="selopacity_Volt"  targetAttr="value" to="1" dur="0ms" ease="in" />
  <animate  trigger="{@m_ds_ML.VIMPBM_BatteryVoltage.valstr}" triggerOn="65024" targetNode="selopacity_Volt"  targetAttr="value" to="0" dur="0ms" ease="in" />
  <animate  trigger="{@m_ds_ML.VIMPBM_BatteryCurrent.valstr}" triggerOn="*"  targetNode="selopacity_Amp" targetAttr="value" to="1" dur="0ms" ease="in" />
  <animate  trigger="{@m_ds_ML.VIMPBM_BatteryCurrent.valstr}" triggerOn="65024" targetNode="selopacity_Amp"  targetAttr="value" to="0" dur="0ms" ease="in" />
  <dockLayout id="item" width="{@csm.screenWidth}" height="{@csm.screenHeight}" depth="-1" clip="false" xmlns="http://www.tat.se/kastor/kml" >
    <dockLayout id="list_item_title" x="0" width="{@csm.screenWidth}" height="{@csm.Gearselection.text_heght-@csm.pageVisualCP_y}">
      <text id="volt_amp_text" x="0" ellipsize="false" font="{@csm.listUnselFont}" color="{@csm.itemUnselColor}" dockLayout.halign="left" dockLayout.valign="bottom" string="{ItemTitle}" />            
    </dockLayout>    
    <dockLayout id="gear_layout" y="0" x="0" width="{@csm.screenWidth}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <image id="battery_image" x="0" dockLayout.halign="left" dockLayout.valign="bottom" opacity="1" src="{@m_MenuModel.Gauges.VoltAmpereMeter.image}"/>
    </dockLayout>
    <!--DockLayout for Voltage Value-->
    <dockLayout id="volt_value" x="0" width="{@csm.VoltAmpereMeter.volt_value_x-@csm.VoltAmpereMeter.List_x}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <text id="volt_value_text" x="0" opacity="{selopacity_Volt*selopacity_Amp}" ellipsize="false" font="{@csm.listUnselFont}" color="{@csm.itemSelColor}" dockLayout.halign="right" dockLayout.valign="bottom" string="{@m_ds_ML.VIMPBM_BatteryVoltage.valstr}" >     
      </text>
    </dockLayout>   
    <!--DockLayout for Voltage Unit-->
    <dockLayout id="volt_unit" x="{@csm.VoltAmpereMeter.volt_unit_x-@csm.VoltAmpereMeter.List_x}" width="{@csm.screenWidth}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <text id="volt_unit_text" x="0" opacity="{selopacity_Volt*selopacity_Amp}" ellipsize="false" font="{@csm.listUnselFont}" color="{@csm.itemSelColor}" dockLayout.halign="left" dockLayout.valign="bottom" string="V" >         
      </text>
    </dockLayout>
    <!--DockLayout for Ampere Value-->
    <dockLayout id="ampere_value" x="0" width="{@csm.VoltAmpereMeter.ampere_value_x-@csm.VoltAmpereMeter.List_x}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <text id="ampere_value_text" x="0" opacity="{selopacity_Amp*selopacity_Volt}" ellipsize="false" font="{@csm.listUnselFont}" color="{@csm.itemSelColor}" dockLayout.halign="right" dockLayout.valign="bottom" string="{@m_ds_ML.VIMPBM_BatteryCurrent.valstr}" >   
      </text>
    </dockLayout>
    <!--DockLayout for Ampere Unit-->
    <dockLayout id="ampere_unit" x="{@csm.VoltAmpereMeter.ampere_unit_x-@csm.VoltAmpereMeter.List_x}" width="{@csm.screenWidth}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <text id="ampere_unit_text" x="0" opacity="{selopacity_Amp*selopacity_Volt}" ellipsize="false" font="{@csm.listUnselFont}" color="{@csm.itemSelColor}" dockLayout.halign="left" dockLayout.valign="bottom" string="A" >           
      </text>
    </dockLayout>
    <!--DockLayout for containing Data Not Available text-->
    <dockLayout id="no_data_textline" x="{@csm.VoltAmpereMeter.List_x1-@csm.VoltAmpereMeter.List_x}" width="{@csm.screenWidth}" height="{@csm.vmImage_y_gearselection-@csm.pageVisualCP_y}">
      <text id="no_data_text" x="0" opacity="{1-(selopacity_Amp*selopacity_Volt)}" ellipsize="false" font="{@csm.listSelFont}" color="{@csm.itemSelColor}" dockLayout.halign="left" dockLayout.valign="bottom" string="{text1}" >           
      </text>
    </dockLayout>
    <!--<rect id="test_rect1" x="{151-28}" y="0" width="1" height="240" opacity="1" fill="#00ff00" />
              <rect id="test_rect1" x="{237-28}" y="0" width="1" height="240" opacity="1" fill="#00ff00" />
              <rect id="test_rect1" x="{160-28}" y="0" width="1" height="240" opacity="1" fill="#00ff00" />
              <rect id="test_rect1" x="{246-28}" y="0" width="1" height="240" opacity="1" fill="#00ff00" />
              <rect id="test_rect8" x="0" y="{161-40}" width="320" height="1" opacity="1" fill="#00ff00" />
              <rect id="test_rect1" x="{109-28}" y="0" width="1" height="240" opacity="1" fill="#00ff00" />-->
  </dockLayout>  
</dockLayout>

Answer 1

你能提供一个字符串的测试用例，它的第一个匹配是“狗慢”吗？顺便说一句，虽然我不知道这对性能是否重要，但RE中有一个不精确的地方--它匹配{@csm开始之后的任何单个字符，而不仅仅是一个点；也许更好的表达式(可能更快，因为它没有使任何点“可选”)可能是：

r'\{@csm((?:\.\w+)+)\}'

Answer 2

我不是一个真正的正则表达式专家，但这可能是由于比赛结束时的支撑。您可以尝试匹配r"\{@csm.((?:[a-zA-Z0-9_]+\.?)+)"，然后手动检查末尾是否出现右大括号。

Answer 3

你可能需要给出一个更好的例子来说明什么是慢的。对于包含匹配和不匹配的内容的相当长的字符串：

x="".join(['{@csm.foo.bar-%d}\n{@csm.foo.%dx.baz}\n' % (a,a)
            for a in xrange(10000)])
mymatch=r"\{@csm.((?:[a-zA-Z0-9_]+\.?)+)\}"

for y in re.finditer(mymatch,x):
    print y.group(0)

工作得很好，但是如果你有一个足够长的字符串，而你对它的搜索很糟糕，你可能会有问题。