How do I check a string exist in a file using PowerShell?

Question

I have a 1st text file looks like this : 12AB34.US. The second text file is CD 34 EF. I want to find my 2nd text file exist or not in the 1st text file.

I tried to cut 3 characters last in the first text file (.US). Then I split to each 2 characters (because the 2nd text file consist of 2 characters). Then, I tried this code, and it always return "Not Found".

$String = Get-Content "C:\Users\te2.txt"
$Data = Get-Content "C:\Users\Fixed.txt"
$Split = $Data -split '(..)'

$Cut = $String.Substring(0,6)

$String_Split = $Cut -split '(..)'
$String_Split

$Check= $String_Split | %{$_ -match $Split}
if ($Check-contains $true) {
    Write-Host "0"
} else {
     Write-Host "1"
}

This is very unclear to me.. The only characters in text2 that are also in text1 are the numbers 34. Is that already enough for you to call it a 'match'? — Theo
– Theo, Commented Mar 5, 2019 at 10:16
Ya. But I am not sure another function to check the data in text2 exist in text1. @Theo — Job
– Job, Commented Mar 8, 2019 at 6:25

Mathias R. Jessen · Accepted Answer · 2019-03-05 19:35:16Z

1

There are a number of problems with your current approach.

The 2-char groups don't align:

    # strings split into groups of two
    '12'    'AB'    '34'        # first string
    'CD'    ' 3'    '4 '        # second string

When you test multiple strings with -match, you need to
1. escape the input string to avoid matchings on meta characters (like .), and
2. place the collection on the left-hand side of the operator, the pattern on the right:

$Compare = $FBString_Split | % {$Data_Split -match [regex]::Escape($_)}
if ($Compare -contains $true) {
    Write-Host "Found"
} else {
     Write-Host "Not Found"
}

For a more general solution to find out if any substring of N chars of one string is also a substring of another, you could probably do something like this instead:

$a = '12AB34.US'
$b = 'CD 34 EF'

# we want to test all substrings of length 2
$n = 2

$possibleSubstrings = 0..($n - 1) | ForEach-Object {
    # grab substrings of length $n at every offset from 0 to $n
    $a.Substring($_) -split "($('.'*$n))" | Where-Object Length -eq $n |ForEach-Object {
        # escape the substring for later use with `-match`
        [regex]::Escape($_)
    }
} |Sort-Object -Unique

# We can construct a single regex pattern for all possible substrings:
$pattern = $possibleSubstrings -join '|'

# And finally we test if it matches
if($b -match $pattern){
    Write-Host "Found!"
}
else {
    Write-Host "Not found!"
}

This approach will give you the correct answer, but it'll become extremely slow on large inputs, at which point you may want to look at non-regex based strategies like Boyer-Moore

edited Mar 5, 2019 at 19:35

answered Mar 5, 2019 at 13:58

Mathias R. Jessen

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1 Comment

Job Over a year ago

It always found, even when I change the $b to : C34DEF . But my expectation $b will divide to each 2 characters when it process to compare with $a.

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