Split and regex match with Powershell - powershell

Question

Say I have a filename string, something like:
test_ABC_19000101_010101.987.txt,
Where "test" could be any combination of white space, characters, numbers, etc. I wish to extract the 19000101_010101 part (date and time) with Powershell. Currently I am assigning -split "_ABC_" to a variable and taking the second element of the array. I am then splitting this string subsequent times. Is there a way to accomplish this in one go?
PS
"_ABC_" is constant, occurring unchanged in all instances of filename(s).

Answer 1

A more concise - albeit perhaps more obscure - alternative to Santiago Squarzon's helpful answer:
# Construct a regex that consumes the entire file name while
# using capture groups for the parts of interest.
$re = '.+_ABC_(\d{4})(\d{2})(\d{2})_(\d{2})(\d{2})(\d{2})\.(\d{3})\..+'
[datetime] (
# In the replacement string, use $1, $2, ... to refer to what the
# first, second, ... capture group captured.
'test_ABC_19000101_010101.987.txt' -replace $re, '$1-$2-$3T$4:$5:$6.$7'
)
Output:
Monday, January 1, 1900 1:01:01 AM
The -replace operation results in string '1900-01-01T01:01:01.987', which is a (culture-invariant) format that you can use as-is with a [datetime] cast.
Note that with a Get-ChildItem call as input you could slightly simplify the regex by providing $_.BaseName rather than $_.Name as the -replace LHS, which obviates the need to also match the extension (.\.+) in the regex.
An aside re the [datetime] cast: [datetime] '...' results in a [datetime] instance that is an unspecified timestamp (its .Kind property value is Unspecified), i.e. it is undefined whether it represents as Local or a Utc timestamp.
To get a Local timestamp, use
[datetime]::Parse('...', [cultureinfo]::InvariantCulture, 'AssumeLocal')
(use 'AssumeLocal, AdjustToUniversal' to get a Utc timestamp).
Alternatively, you can cast to [datetimeoffset] - a type that is generally preferable to [datetime] - which interprets a string cast to it as local by default. (You can then access its .LocalDateTime / .UtcDateTime properties to get Local / Utc [datetime] instances).

Answer 2

This regex seems an overkill but I think it should work, as long as _ABC_ is constant and there is a _ to separate the date from the time and a . to separate time from milliseconds:
$re = [regex]'(?<=_ABC_)(?<date>\d*)_(?<time>\d*)\.(?<millisec>\d*)(?=\.)'
#'
test_ABC_19000101_010101.987.txt
t' az# 0est_ABC_20000101_090101.123.txt
tes8as712t_ABC_21000101_080101.456.txt
te098d $st_ABC_22000101_070101.789.txt
[test]_ABC_23000101_060101.101.txt
t?\est_ABC_24000101_050101.112.txt
'# -split '\r?\n' | ForEach-Object {
$groups = $re.Match($_).Groups
$date = $groups['date']
$time = $groups['time']
$msec = $groups['millisec']
[datetime]::ParseExact(
"$date $time $msec",
"yyyyMMdd HHmmss fff",
[cultureinfo]::InvariantCulture
)
}
See https://regex101.com/r/8oSpqf/1 for details.

Answer 3

If there will never be multiple sequences in the filename that appear as the timestamp (8 digits, _, 6 digits, then you could match on that pattern of digits.
PS C:\> 'test_ABC_19000101_010101.987.txt' -match '^.*ABC_(\d{8}_\d{6})\..*'
True
PS C:\> $Matches
Name Value
---- -----
1 19000101_010101
0 test_ABC_19000101_010101.987.txt
PS C:\> $Matches[1]
19000101_010101
You would use the filename instead of the explicit string.
If you want to get a [System.DateTime] from it:
PS C:\> [datetime]::ParseExact($Matches[1], 'yyyyMMdd_HHmmss', $null)
Monday, January 1, 1900 01:01:01