GNU BRE/ERE cheatsheet and differences between grep, sed and awk

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2021-06-11 12:00:09

This blog post covers Basic Regular Expressions (BRE) and Extended Regular Expressions (ERE) syntax supported by GNU grep, sed and awk. You'll also learn the differences between these tools — for example, awk doesn't support backreferences within regexp definition (i.e. the search portion).

In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{, \|, \(, and \).

grep and sed support BRE by default and enables ERE when -E option is used. awk supports only ERE. Assume ERE for descriptions in this post unless otherwise mentioned.

This post is intended as a reference for BRE/ERE flavor of regular expressions. For a more detailed explanation with examples and exercises, see these chapters from my ebooks:

grep also supports -w cli option. It is equivalent to (?<!\w)pattern(?!\w). The three different ways to specify word anchors are not exactly equivalent though, see Word boundary differences section from my book for details and examples.