Linux Basic — Regular Expressions

Regular Expressions

Regular expressions (regex) are how you describe patterns in text. Every serious shell tool — grep, sed, awk, bash itself — speaks at least one regex dialect. This chapter teaches the dialect each tool actually accepts and the idioms that work.

---

What You Will Learn

By the end of this chapter you will be able to:

• Tell BRE (basic) from ERE (extended) regex and pick the right flag
• Use grep -E to filter text with extended patterns
• Match a regex inside Bash with the [[ $str =~ re ]] operator and capture groups via BASH_REMATCH
• Use POSIX character classes ([:alpha:], [:digit:], …) for portability
• Apply regex with sed -E for substitution and awk for pattern actions

---

Topics in This Chapter

#	Topic	Description
1	grep -E in Scripts	extended regex with grep
2	Bash =~ Operator	regex inside [[ ]], BASH_REMATCH
3	POSIX Character Classes	[:digit:], [:alpha:], etc.
4	sed Regex in Scripts	substitution, anchors, capture refs
5	awk Regex Patterns	pattern-action programs
6	Practice Exercises	Hands-on tasks

---

BRE vs ERE in One Table

Construct	BRE (grep, sed)	ERE (grep -E, sed -E, awk, Bash =~)
Group	\( \)	( )
Alternation	not supported	\|
One-or-more	\+ (GNU ext.)	+
Zero-or-one	\? (GNU ext.)	?
{n,m} quantifier	\{n,m\}	{n,m}
. * ^ $ [ ]	same in both	same in both

Rule of thumb: when in doubt, use ERE (grep -E, sed -E). It’s what awk and Bash’s =~ use, so the same pattern works across all three.

---

A Tiny Pattern Vocabulary

Pattern	Meaning
^abc	line starts with abc
abc$	line ends with abc
.	any single character
[abc]	one of a, b, c
[^abc]	not a, b, or c
\d	not POSIX — use [0-9] or [[:digit:]]
a*	zero-or-more a
a+	one-or-more a (ERE)
a?	zero-or-one a (ERE)
a{2,4}	2 to 4 as (ERE)
(foo\|bar)	foo or bar (ERE)

---

Next → grep -E in Scripts