Grep & Regex

Grep

A program for finding matching patterns. Follows $ grep <pattern> <file> or as part of a pipe: $ find . -name *.txt | grep taxes

Common Flags:

• -i : makes it case insensitive

• -r : makes the search recursive (searches directories)

• -v : makes the search find all instances where there is not a pattern match

• -w : makes the search match on a word rather than any pattern match

Regular Expressions (Regex)

Regex is passed into grep like so grep '<expression>' file

• ^ - match expression at the start of a line, as in ^A

• $ - match expression at the end of a line

• \ - Turn off the special meaning of the next character and treat it as a string, as in \* or \^

• [ ] - Match any one of the enclosed characters, as in [aeiou] or a range like [0-9] or [a-zA-Z]

• [^ ] - Match any one character except those enclosed in [ ] as in [^0-9]

• . Match a single character of any value except EOL

• * - Match zero or more of the preceding character or expression

• \{x,y\} - Match x to y occurences of the preceding

• \{x\} - Match exactly x occurrences of the preceding

• \{x,\} - Match x or more occurences of the preceding

Grep Examples

• grep linux files - Search files for lines with the word linux

• grep '^linux' files with linux at the start of a line

• grep 'linux$' files with linux at the end of a line

• grep '^linux$' files with lines containing only linux

• grep '\^s' files lines starting with ^s (\ is an escape for that character)

• grep '[Ll]inux' files search for either Linux or linux

• grep 'B[oO][bB]' files search for BOB, Bob, BOb or BoB

• grep '^$' files search for blank files

• grep '[0-9][0-9]' files search for pairs of numeric digits