What you have (look-aheads) are available only in the PCRE regex flavor which is supported only in GNU grep with its -P flag.. The name grep stands for “global regular expression print”. You seem to have defined the right regex, but not set the sufficient flags in command-line for grep to understand it. Regular Expressions. The utilities allow the user to search text files for lines that match a regular expression (regexp). As we saw in Getting started with regular expressions: An example, the -v option reverses those actions, so that the lines with matches are discarded. Tell grep not to bother about encoding, and consider one byte as one char. Introduction. Regular Expression provides an ability to match a “string of text” in a very flexible and concise manner. It's easy to formulate a regex using what you want to match. Let’s look to see if emails are contained in our files. There is a workaround in your case. The patterns used here are not the only way to construct a RegEx search, and there may be easier ways. Regular expressions, aka "regex", are patterns that describe sets of strings. Since your terminal doesn't recognise the char either, you get a . In other words, they allow you to match complex patterns with grep, not just exact matches. Prerequisite: grep. One easy way to exclude text from a match is negative lookbehind: w+b(?" input. PowerShell Grep (Select-String) is a pretty advanced cmdlet. In the following example, we will search for a word webservertalk, use (\s|$) to search a specified word ending with white space and use /b to match the empty string at the edge of a word.. grep -E "\bwebservertalk(\s|$)" file.txt When grep encounters a byte sequence that is not a valid char in the expected encoding, it cannot recognise it as a character, the line doesn't match, it's output. Negative matching using grep (match lines that do not contain foo) 368. 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