Another handy grep trick you can use is the -o (only matching) option. This is a grep trick—it’s not part of the regex functionality. The power of regular expressions comes from its use of metacharacters, which are special characters (or sequences of characters) that are used to represent something else. For instance, in a regular expression the metacharacter ^ means "not". Stating a regex in terms of what you don't want to match is a bit harder. For instance, with A*, the engine starts out matching zero characters, since * allows the engine to match "zero or more". You're not limited to searching for simple strings but also patterns within patterns. Since 3.0, Bash supports the =~ operator to the [[ keyword. Unix/Linux find command “patterns” FAQ: How do I find files or directories that don’t match a specific pattern (files not matching a regex pattern, or filename pattern)?. This operator matches the string that comes before it against the regex pattern that follows it. EDIT: Here are some strings that should match the regex The syntax for using regular expressions to match lines in awk is: word ~ /match/ The inverse of that is not matching a pattern: word !~ /match/ If you haven't already, create the sample file from our previous article: We type the following: grep -E -n 'o' geeks.txt. So, while "a" means "match lowercase a", "^a" means "do not match lowercase a". 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. Match everything except for specified strings . One easy way to exclude text from a match is negative lookbehind: w+b(?
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