regex

regex — regular expression pattern matching syntax

Regular expressions (regex) are patterns used to match character combinations in strings. They are supported by many commands including grep, sed, awk, perl, and programming languages.

.: Match any single character
\*: Match zero or more of preceding element
^: Match start of line
$: Match end of line
[abc]: Match any character in brackets
[^abc]: Match any character not in brackets
[a-z]: Match character range
\\: Escape special character

+: Match one or more of preceding element
?: Match zero or one of preceding element
|: Alternation (OR)
(...): Grouping
{n}: Match exactly n times
{n,}: Match n or more times
{n,m}: Match between n and m times

\d: Digit (PCRE) — equivalent to [0-9]
\w: Word character — [a-zA-Z0-9_]
\s: Whitespace — [ \t\n\r\f]
\D, \W, \S: Negated versions

[[:alpha:]]: Alphabetic characters
[[:digit:]]: Digits
[[:alnum:]]: Alphanumeric
[[:space:]]: Whitespace
[[:upper:]]: Uppercase letters
[[:lower:]]: Lowercase letters

^: Start of line/string
$: End of line/string
\b: Word boundary (PCRE)
\B: Non-word boundary (PCRE)

Different tools support different regex flavors: BRE (basic), ERE (extended), PCRE (Perl-compatible). Use appropriate flags: grep -E for ERE, grep -P for PCRE.
Escape sequences and metacharacters vary between flavors. Test patterns with your specific tool.
Greedy vs. non-greedy: \* and + are greedy by default. Use **\*? and +?** for non-greedy matching (PCRE).

grep(1), sed(1), awk(1), perlre(1), regex(7)