metacharacters in regular expression grep command

Metacharacters in regular expression grep command series#

If a match occurs, then the entire line is printed out. There are quite a few character classes that are very commonly used in regular expressions, and these are provided as named classes. Here, the literal regular expression her is determined by grep as a regular expression matching argument to be compared with each line in the file input-file. Here, “expressions” is the only all-lowercase 11-character string in the file so this is the only line printed as the output. In basic regular expressions, the meta-characters ?, +, ' testfile GNU Grep has two regular expression feature sets: Basic and Extended. In fact, most varieties of regular expressions are quite similar, but have differences in escapes, meta-characters, or special operators. GNU Grep uses the GNU version of regular expressions, which is very similar (but not identical) to POSIX regular expressions. Usually, regular expressions are included in the Grep command in the following format: grep Regexes enhance the ability to meaningfully process text content, especially when combined with other commands. Regular expressionsĪ regular expression, often shortened to “regex” or “regexp”, is a way of specifying a pattern (a particular set of characters or words) in text that can be applied to variable inputs to find all occurrences that match the pattern.

Metacharacters in regular expression grep command series#

Grep also accepts inputs (usually via a pipe) from another command or series of commands. First, Grep can be used to search a given file or files on a system (including a recursive search through sub-folders). There are two ways to provide input to Grep, each with its own particular uses. In such combined implementations, Grep may also behave differently depending on the name by which it is invoked, allowing fGrep, eGrep, and Grep to be links to the same program. These variants are embodied in most modern Grep implementations as command-line switches (and standardised as -E and -F in POSIX.2). The latter searches for any of a list of fixed strings, using the Aho-Corasick algorithm.

The tool has its roots in an extended regular expression syntax that was added to UNIX after Ken Thompson’s original regular expression implementation.

While most everyday uses of the command are simple, there are a variety of more advanced uses that most people don’t know about - including regular expressions and more, which can become quite complicated. Grep finds a string in a given file or input, quickly and efficiently.