Regular Expressions & grep

Regular Expressions

A regular expression (regex) is a compact pattern language for describing shapes of text, rather than exact literal text - "a line that starts with three digits" instead of needing to spell out every possible three-digit combination. grep is the classic tool for finding lines that match a regex, but the pattern language itself shows up across sed, awk, and many other tools too, which is why it's worth learning as its own topic.

Pattern	Matches
`.`	Any single character
`*`	Zero or more of the previous
`^`	Start of line
`$`	End of line
`[abc]`	Any one of a, b, c
`[^abc]`	Any character except a, b, c
`\{2,4\}`	2 to 4 repetitions (BRE)
`\	`	Alternation (BRE)

A subtlety worth sitting with: . in regex means "any character," not a literal dot - and means "zero or more of whatever came right before it," not "any characters" the way a shell wildcard does. So a.b means "an a, then any number of any characters, then a b" - a very different (and much more powerful) idea than the filename globbing you've used with ls or find.

grep flags: -i ignore case, -v invert (show non-matching lines instead), -r recursive (search every file under a directory), -n show line numbers, -c count matches instead of printing them, -E extended regex, -o print only the matched part of the line (not the whole line), -w match whole words only.

$ grep -E '^[0-9]{3}-[0-9]{4}
  

 contacts.txt   # phone numbers
$ grep -rn 'TODO' src/

The first example reads as: "a line that is entirely three digits, a dash, then four digits, and nothing else" - the ^ and $ anchors ensure it doesn't just match a phone number buried inside a longer line. The second recursively searches every file under src/ for the literal text TODO, printing the file name and line number for each hit.

Tip: There are actually two dialects of regex syntax in play, and it's a genuine, well-known gotcha: BRE (Basic Regular Expressions, grep's default) requires backslashes before {, }, and | to give them their special meaning (\{2,4\}, a\|b). ERE (Extended Regular Expressions, activated with grep -E) uses those characters bare, without backslashes ({2,4}, a|b). egrep is simply a shorthand alias for grep -E. If your {2,4} pattern seems to match literally nothing, check whether you meant to add -E or add the backslashes.