Glossary
Regular expression
A pattern language for matching text, used by grep, sed and awk.
Regular expressions describe text patterns: ^ anchors the start of a line, $ the end, . matches any character, * repeats the previous element, [0-9] a digit, | alternation. grep, sed and awk are built around them.
They are not globs, despite sharing symbols: in a glob * means "any characters", in a regex it means "zero or more of the previous thing". Mixing the two mental models is a classic source of confusion.
See it in practice
$ grep -E "^(error|warn):" app.logAnchored to the line start, matching either word. Extended regex (-E) enables the parentheses and pipe without escaping.
Where you'll meet it
Find lines matching a pattern in files or piped input — the workhorse of text search.
sedStream-edit text: substitute, delete lines, and edit files in place with -i.
awkPull columns out of any output, filter rows, and compute sums — the shell's spreadsheet.
Glob (wildcard)Patterns like *.txt, expanded by the shell before the command runs.