When I run:
$SearchStr = "c:\programFiles\xyz"
Select-String -pattern $SearchStr -path $env:SystemRoot\win.ini
I get an error message stating that the string is "not a valid regular expression"
By contrast, it is working fine when $SearchStr doesn't contain special characters.
Why doesn't it work with special characters such as \?
If you want Select-String to match literals (literal substring matching) rather than (by default) regular expressions, use -SimpleMatch:
Select-String -SimpleMatch -Pattern $SearchStr -Path $env:SystemRoot\win.ini
By contrast, if you need to incorporate a literal into a regular expression, apply [regex]::Escape() to it, as TessellatingHeckler suggests.
E.g., to only find $SearchStr at word boundaries (\b):
Select-String -Pattern ('\b' + [regex]::Escape($SearchStr) + '\b') -Path $env:SystemRoot\win.ini
By default, Select-String interprets the search terms passed to parameter -Pattern as regexes (regular expressions).
In regular expressions, \ has special meaning: it is used as the escape character to modify how the regular-expression engine would otherwise interpret the following character(s).
Therefore, the \ in Windows-style paths such as c:\programFiles\xyz results in interpretation of the \ chars. as the start of an escape sequence.
Using the sample path: \p, if followed by a Unicode character category such as {L}, matches any character in that category; \x, if followed by 2 hexadecimal digits, matches a single character with that code point.
To escape \ itself, so that it is treated literally, double it: \\.
The .NET framework [regex]::Escape() method can do this for you, along with escaping any other characters that have special meaning to the regex engine.
In other words: to embed a literal string in a regular expression, pass it to [regex]::Escape() first.
