Lesson 10.1: Regular expressions: characters, classes and anchors

What a regular expression is and how it differs from shell wildcards, literal characters and metacharacters, the dot, bracket expressions with ranges and negation, POSIX character classes, the anchors ^ and $, and escaping a special character with a backslash. Every example runs through grep.

18 min

You will learn to

Explain what a regular expression describes
Tell regex metacharacters from shell wildcards
Match one character of a set with [...] and exclude with [^...]
Use POSIX classes such as [[:digit:]] and [[:upper:]]
Anchor a pattern to the start or end of a line
Escape . and other metacharacters

Key terms

Regular expression (regex): A pattern that describes a set of strings. A program checks each line (or string) against it.
Literal and metacharacter: Most characters match themselves. Metacharacters have a special meaning: . [ ] ^ $ * \ (and in extended regex also + ? { } ( ) |).
. (dot): Matches any single character.
[...] bracket expression: Matches one character from the list. Ranges like a-z and 0-9 are allowed; [^...] matches one character NOT in the list.
POSIX classes: Named sets used inside brackets: [:digit:] [:alpha:] [:alnum:] [:upper:] [:lower:] [:space:] [:punct:]. Write them with double brackets: [[:digit:]].
Anchors ^ and $: ^ matches the start of a line, $ the end. They match a position, not a character.
Escaping: A backslash before a metacharacter makes it literal: \. \* \[ \$.

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