Match everything except for specified strings . Let's create example file.txt with some text: add character at the beginning of each line Add character at the beginning of each line using sed command. I tried the same thing with egrep, I tried to use it with double and single quotes, but to no better results.When I try ls /directory | grep '^[term] I get all entries beginning with term - as expected.. The annoying issue is that GNU grep (or glibc) supports some PCRE-like atoms, at least \w and \s when interpreting ERE, and in that context they very much are nonstandard. A regular expression is a pattern that describes a set of strings. @DanielFarrell, the standard in this case is what POSIX specifies, and it doesn't know about \d.Though you're right in that PCRE are rather standard, or in the least well-defined. This example shows how to add a character to the beginning of each line using a sed command and bash shell. Regular expressions are constructed analogously to arithmetic expressions by using various operators to combine smaller expressions. (See the perlre man page) They're not supported in the extended regular expressions that Bash uses.. The fundamental building blocks are the regular expressions that match a single character. But if you happen not to have a regular expression implementation with this feature (see Comparison of Regular Expression Flavors), you probably have to build a regular expression with the basic features on your own. Since there are many engines for regex, we will use the shell regex and see the bash power in working with regex. The first rule to know is that regular expression patterns are case sensitive. The -Po flags, instruct grep to interpret the pattern as a Perl-compatible regular expression. First, we need to understand what regex is; then we will see how to use it. The next column, "Legend", explains what the element means (or encodes) in the regex syntax. Edit: I don't want to search for '^$1' but for a dynamically inserted fixed string which should only be matched if it's at the beginning of a line. ... To locate the beginning of a line in a text, use the caret character (^). The \K escape sequence causes what was previously matched (the first three characters) not to be included at the end, then . * matches everything that follows. What happened is this; our first selection group captured the text abcdefghijklmno.Then, given the . !999)\d{3} This example matches three digits other than 999. You could use a look-ahead assertion: (? grep , expr , sed and awk are some of them.Bash also have =~ operator which is named as RE-match operator.In this tutorial we will look =~ operator and use cases.More information about regex command cna be found in the following tutorials. Using awk In ERE, the start of string is represented as ^, and any whitespace character can be matched with [[:space:]], or if you want to just match a space, with a literal space. When I execute ls /directory | grep '[^term]' in Bash I get a regular listing, as if the grep command is ignored somehow. * (any character, 0 or more times) all characters were matched - and this important; to the maximum extent - until we find the next applicable matching regular expression, if any.Then, finally, we matched any letter out of the A-Z range, and this one more times. Linux bash provides a lot of commands and features for Regular Expressions or regex. On each line, in the leftmost column, you will find a new element of regex syntax. \A, \b and \s are Perl for "start of string", "word boundary" and "a whitespace character", respectively. 5. That's what I meant by the $1 . 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