PowerShell:选择匹配项之前的行-使用输入字符串变量时Select-String -Context问题 [英] PowerShell: Select line preceding a match -- Select-String -Context issue when using input string variable

问题描述

我需要在多行字符串变量上返回匹配项之前的一行.

I need return a line preceeding a match on a multi-line string variable.

似乎在将字符串变量用于输入Select-String时，会将整个字符串视为已匹配.因此，Context属性位于字符串的任一端外部"，并且为null.

It seems when using a string variable for the input Select-String considers the entire string as having matched. As such the Context properties are "outside" either end of the string and are null.

考虑以下示例:

$teststring = @"
line1
line2
line3
line4
line5
"@

Write-Host "Line Count:" ($teststring | Measure-Object -Line).Lines #verify PowerShell does regard input as a multi-line string (it does)

Select-String -Pattern "line3" -InputObject $teststring -AllMatches -Context 1,0 | % {
$_.Matches.Value #this prints the exact match
$_.Context #output shows all context properties to be empty 
$_.Context.PreContext[0] #this would ideally output first line before the match
$_.Context.PreContext[0] -eq $null #but instead is null
}

我在这里误会了吗?

匹配"line3"时返回"line2"的最佳方法是什么?

What is the best way to return "line2" when matching for "line3"?

谢谢！

我忽略声明的其他要求: 需要在所有匹配的行上方提供不确定长度的字符串. EG在下面搜索"line3"时，我需要返回"line2"和"line5".

Additional requirements I neglected to state: Needs to provide the line above ALL matched lines for a string of indeterminate length. EG when searching the below for "line3" I need to return "line2" and "line5".

line1
line2
line3
line4
line5
line3
line6

推荐答案

Select-String 对输入的数组进行操作，因此而不是单行，多行字符串，您必须提供行数组 ，以使-Context和-AllMatches能够按预期工作:

Select-String operates on arrays of input, so rather than a single, multiline string you must provide an array of lines for -Context and -AllMatches to work as intended:

$teststring = @"
line1
line2
line3
line4
line5
line3
line6
"@

$teststring -split '\r?\n' | Select-String -Pattern "line3" -AllMatches -Context 1,0 | % {
  "line before:  " + $_.Context.PreContext[0]
  "matched part: " + $_.Matches.Value  # Prints the what the pattern matched
}

这将产生:

line before:  line2
matched part: line3
line before:  line5
matched part: line3

• $teststring -split '\r?\n'将多行字符串分成行数组:

  • $teststring -split '\r?\n' splits the multi-line string into an array of lines:

    • 注意:您的此处文档使用的换行符顺序(仅LF相对于CRLF)取决于随附的脚本文件； regex \r?\n处理这两种样式.
    • Note: What line-break sequences your here-document uses (LF-only vs. CRLF) depends on the enclosing script file; regex \r?\n handles either style.

    请注意，使用管道提供Select-String的输入至关重要.如果使用-InputObject，则该数组将被强制返回为单个字符串.

    Note that it is crucial to use the pipeline to provide Select-String's input; if you used -InputObject, the array would be coerced back to a single string.

    Select-String很方便，但是慢.
    特别是对于已经在内存中的单个字符串，使用.NET Framework的

    Select-String is convenient, but slow.
    Especially for a single string already in memory, a solution using the .NET Framework's [Regex]::Matches() method will perform much better, though it is more complex.

    请注意，PowerShell自己的-match和-replace运算符是基于同一.NET类构建的，但是并未公开其所有功能；请参见-match-不会在自动$Matches变量中报告捕获组-在这里不是一个选项，因为它只会返回 1 匹配.

    Note that PowerShell's own -match and -replace operators are built on the same .NET class, but do not expose all of its functionality; -match - which does report capture groups in the automatic $Matches variable - is not an option here, because it only ever returns 1 match.

    ^{以下内容与} mjolinor的答案答案基本相同，但已纠正了一些问题[1] ].

    ^{The following is essentially the same approach as in mjolinor's answer answer, but with several problems corrected[1].}

    # Note: The sample string is defined so that it contains LF-only (\n)
    #       line breaks, merely to simplify the regex below for illustration.
    #       If your script file use LF-only line breaks, the 
    #       `-replace '\r?\n', "`n" call isn't needed.
    $teststring = @"
    line1
    line2
    line3
    line4
    line5
    line3
    line6
    "@ -replace '\r?\n', "`n" 
    
    [Regex]::Matches($teststring, '(?:^|(.*)\n).*(line3)') | ForEach-Object { 
      "line before:  " + $_.Groups[1].Value
      "matched part: " + $_.Groups[2].Value
    }
    

    • Regex (?:^|(.*)\n).*(line3)使用2个捕获组((...))捕获要匹配的行(匹配的部分)和之前的行((?:...)是辅助 non 捕获优先级所需的组):

      • Regex (?:^|(.*)\n).*(line3) uses 2 capture groups ((...)) to capture both the (matching part of) the line to match and the line before ((?:...) is an auxiliary non-capturing group that is needed for precedence):

        • (?:^|(.*)\n)匹配字符串(^)或(|)开头的任何-可能为空的非换行字符序列(.*)后跟换行符(\n) ;这样可以确保在前一行没有 的情况下也找到要匹配的行(即，要匹配的行是第一个).
        • (line3)是定义要匹配的行的组；它以.*开头以匹配问题中的行为，即使仅是行的 part ，也可以在其中找到模式line3.
          • 如果只希望匹配 full 行，请使用以下正则表达式代替:
            (?:^|(.*)\n)(line3)(?:\n|$)
          • (?:^|(.*)\n) matches either the very start of the string (^) or (|) any - possibly empty - sequence of non-newline characters (.*) followed by a newline (\n); this ensures that the line to match is also found when there is no preceding line (i.e., of the line to match is the first one).
          • (line3) is the group defining the line to match; it is preceded by .* to match the behavior in the question, where pattern line3 is found even it is only part of a line.
            • If you want only full lines to match, use the following regex instead:
              (?:^|(.*)\n)(line3)(?:\n|$)

            [Regex]::Matches()查找所有 匹配项，并将其作为

            [Regex]::Matches() finds all matches and returns them as a collection of System.Text.RegularExpressions.Match objects, which the ForEach-Object cmdlet call can then operate on to extract the capture-group matches ($_.Groups[<n>].Value).

            ^{[1]撰写本文时:
            -不需要匹配两次-不需要包含if ($teststring -match $pattern) { ... }.
            -不需要内联选项(?m)，因为默认情况下.不会 匹配换行符 .
            -(.+?)仅捕获非空行(并且不需要?(非贪婪的量词)).
            -如果感兴趣的行是第一行-即，如果在之前没有行，则不会匹配.}

            <sup>[1] As of this writing:
            - There is no need to match twice - the enclosing if ($teststring -match $pattern) { ... } is unnecessary.
            - Inline option (?m) is not needed, because . does not match newlines by default.
            - (.+?) captures only nonempty lines (and ?, the non-greedy quantifier, is not needed).
            - If the line of interest is the first line - i.e., if there's no line before, it won't be matched.</sup>

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