Master grep, egrep, and fgrep: Essential Linux Text Filtering Techniques

1. Differences between grep, egrep, and fgrep

grep supports standard regular expressions, while egrep (or grep -e) supports extended regular expressions. fgrep does not support regular expressions at all, offering faster exact‑string matching.

2. Commonly used options

--color=auto : Enable syntax highlighting (default via alias).

-v : Invert match (select non‑matching lines).

-o : Show only the matched part of the line.

-i : Case‑insensitive matching.

-E : Use extended regular expressions (supported by egrep).

-A # : Show # lines after a match.

-B # : Show # lines before a match.

-C # : Show # lines surrounding a match.

3. Common character classes

[:digit:]

– all digits [:lower:] – all lowercase letters [:upper:] – all uppercase letters [:space:] – whitespace characters [:punct:] – punctuation symbols [:alpha:] – all alphabetic characters [:alnum:] – all letters and digits

4. Metacharacters in basic regular expressions

. : Matches any single character.

[] : Matches any one character inside the brackets.

[^] : Matches any character not listed inside the brackets.

* : Matches the preceding element zero or more times.

? : Matches the preceding element zero or one time.

{m,n} : Matches the preceding element at least m and at most n times.

{m} : Matches the preceding element exactly m times.

{0,n} : Matches up to n occurrences.

{m,} : Matches at least m occurrences.

^ : Anchors the match to the start of a line.

$ : Anchors the match to the end of a line.

\< : Anchors to the beginning of a word.

\> : Anchors to the end of a word.

\b : Word boundary (equivalent to \< or \>).

^$ : Matches an empty line.

5. Grouping, alternation, and backreferences

\( … \) : Groups a sub‑pattern, allowing it to be referenced later.

\n : Refers to the n ‑th captured group.

| : Alternation (logical OR) between patterns.

6. Extended regular expressions

Extended regex shares most metacharacters with basic regex but does not require escaping for most of them. The + quantifier matches the preceding element one or more times, making patterns more concise and readable.