How to compare two sequential strings in a file - powershell

Question

I have a big file consists of "before" and "after" cases for every item as follows:
case1 (BEF) ACT
(AFT) BLK
case2 (BEF) ACT
(AFT) ACT
case3 (BEF) ACT
(AFT) CLC
...
I need to select all of the strings which have (BEF) ACT on the "first" string and (AFT) BLK on the "second" and place the result to a file.
The idea is to create a clause like
IF (stringX.LineNumber consists of "(BEF) ACT" AND stringX+1.LineNumber consists of (AFT) BLK)
{OutFile $stringX+$stringX+1}
Sorry for the syntax, I've just starting to work with PS :)
$logfile = 'c:\temp\file.txt'
$matchphrase = '\(BEF\) ACT'
$linenum=Get-Content $logfile | Select-String $matchphrase | ForEach-Object {$_.LineNumber+1}
$linenum
#I've worked out how to get a line number after the line with first required phrase
Create a new file with a result as follows:
string with "(BEF) ACT" following with a string with "(AFT) BLK"

Answer 1

Select-String -SimpleMatch -CaseSensitive '(BEF) ACT' c:\temp\file.txt -Context 0,1 |
ForEach-Object {
$lineAfter = $_.Context.PostContext[0]
if ($lineAfter.Contains('(AFT) BLK')) {
$_.Line, $lineAfter # output
}
} # | Set-Content ...
-SimpleMatch performs string-literal substring matching, which means you can pass the search string as-is, without needing to escape it.
However, if you needed to further constrain the search, such as to ensure that it only occurs at the end of a line ($), you would indeed need a regular expression with the (implied) -Pattern parameter: '\(BEF\) ACT$'
Also note PowerShell is generally case-insensitive by default, which is why switch -CaseSensitive is used.
Note how Select-String can accept file paths directly - no need for a preceding Get-Content call.
-Context 0,1 captures 0 lines before and 1 line after each match, and includes them in the [Microsoft.PowerShell.Commands.MatchInfo] instances that Select-String outputs.
Inside the ForEach-Object script block, $_.Context.PostContext[0] retrieves the line after the match and .Contains() performs a literal substring search in it.
Note that .Contains() is a method of the .NET System.String type, and such methods - unlike PowerShell - are case-sensitive by default, but you can use an optional parameter to change that.
If the substring is found on the subsequent line, both the line at hand and the subsequent one are output.
The above looks for all matching pairs in the input file; if you only wanted to find the first pair, append | Select-Object -First 2 to the Select-String call.

Answer 2

Another way of doing this is to read the $logFile in as a single string and use a RegEx match to get the parts you want:
$logFile = 'c:\temp\file.txt'
$outFile = 'c:\temp\file2.txt'
# read the content of the logfile as a single string
$content = Get-Content -Path $logFile -Raw
$regex = [regex] '(case\d+\s+\(BEF\)\s+ACT\s+\(AFT\)\s+BLK)'
$match = $regex.Match($content)
($output = while ($match.Success) {
$match.Value
$match = $match.NextMatch()
}) | Set-Content -Path $outFile -Force
When used the result is:
case1 (BEF) ACT
(AFT) BLK
case7 (BEF) ACT
(AFT) BLK
Regex details:
( Match the regular expression below and capture its match into backreference number 1
case Match the characters “case” literally
\d Match a single digit 0..9
+ Between one and unlimited times, as many times as possible, giving back as needed (greedy)
\s Match a single character that is a “whitespace character” (spaces, tabs, line breaks, etc.)
+ Between one and unlimited times, as many times as possible, giving back as needed (greedy)
\( Match the character “(” literally
BEF Match the characters “BEF” literally
\) Match the character “)” literally
\s Match a single character that is a “whitespace character” (spaces, tabs, line breaks, etc.)
+ Between one and unlimited times, as many times as possible, giving back as needed (greedy)
ACT Match the characters “ACT” literally
\s Match a single character that is a “whitespace character” (spaces, tabs, line breaks, etc.)
+ Between one and unlimited times, as many times as possible, giving back as needed (greedy)
\( Match the character “(” literally
AFT Match the characters “AFT” literally
\) Match the character “)” literally
\s Match a single character that is a “whitespace character” (spaces, tabs, line breaks, etc.)
+ Between one and unlimited times, as many times as possible, giving back as needed (greedy)
BLK Match the characters “BLK” literally
)

Answer 3

My other answer completes your own Select-String-based solution attempt. Select-String is versatile, but slow, though it is appropriate for processing files too large to fit into memory as a whole, given that it processes files line by line.
However, PowerShell offers a much faster line-by-line processing alternative: switch -File - see the solution below.
Theo's helpful answer, which reads the entire file into memory first, will probably perform best overall, depending on file size, but it comes at the cost of increased complexity, due to relying heavily on direct use of .NET functionality.
$(
$firstLine = ''
switch -CaseSensitive -Regex -File t.txt {
'\(BEF\) ACT' { $firstLine = $_; continue }
'\(AFT\) BLK' {
# Pair found, output it.
# If you don't want to look for further pairs,
# append `; break` inside the block.
if ($firstLine) { $firstLine, $_ }
# Look for further pairs.
$firstLine = ''; continue
}
default { $firstLine = '' }
}
) # | Set-Content ...
Note: The enclosing $(...) is only needed if you want to send the output directly to the pipeline to a cmdlet such as Set-Content; it is not needed for capturing the output in a variable: $pair = switch ...
-Regex interprets the branch conditionals as regular expressions.
$_ inside a branch's action script block ({ ... } refers to the line at hand.
The overall approach is:
$firstLine stores the 1st line of interest once found, and when the 2nd line's pattern is found and $firstLine is set (is nonempty), the pair is output.
The default handler resets $firstLine, to ensure that only two consecutive lines that contain the strings of interest are considered.