Pattern matching

This section describes the pattern matching notation used in the shell. Patterns are used in pathname expansion, case commands, and parameter expansion modifiers.

Literals

A literal is a character that matches itself. For example, the pattern a matches the character a. All characters are literals except for the special characters described below.

Quoting

Quoting makes a special character behave as a literal. See the Quoting section for details. Additionally, for unquoted parts of a pattern produced by parameter expansion, command substitution, or arithmetic expansion, backslashes escape the following character, but such backslashes are not subject to quote removal.

In this example, no pathname expansion occurs because the special characters are quoted:

$ echo a\*b
a*b
$ asterisk='*'
$ echo "$asterisk"
*
$ quoted='a\*b'
$ echo $quoted
a\*b

Special characters

The following characters have special meanings in patterns:

• ? – Matches any single character.
• * – Matches any number of characters, including none.
• [...] – Matches any single character from the set of characters inside the brackets. For example, [abc] matches a, b, or c. Ranges can be specified with a hyphen, like [a-z] for lowercase letters.
• [!...] and [^...] – Matches any single character not in the set of characters inside the brackets. For example, [!abc] matches any character except a, b, or c.

The [^...] form is not supported in all shells; prefer using [!...] for compatibility.

$ echo ?????? # prints all six-character long filenames
Videos
$ echo Do* # prints all files starting with Do
Documents Downloads
$ echo [MP]* # prints all files starting with M or P
Music Pictures
$ echo *[0-9] # prints all files ending with a digit
foo.bak.1 foo.bak.2 bar.bak.3

Special elements in brackets

Bracket expressions […] can include special elements:

• Character classes: [:class:] matches any character in the specified class. Available classes:

  • [:alnum:] – Alphanumeric characters (letters and digits)
  • [:alpha:] – Alphabetic characters (letters)
  • [:blank:] – Space and tab characters
  • [:cntrl:] – Control characters
  • [:digit:] – Digits (0-9)
  • [:graph:] – Printable characters except space
  • [:lower:] – Lowercase letters
  • [:print:] – Printable characters including space
  • [:punct:] – Punctuation characters
  • [:space:] – Space characters (space, tab, newline, etc.)
  • [:upper:] – Uppercase letters
  • [:xdigit:] – Hexadecimal digits (0-9, a-f, A-F)

  $ echo [[:upper:]]* # prints all files starting with an uppercase letter
  Documents Downloads Music Pictures Videos
  $ echo *[[:digit:]~] # prints all files ending with a digit or tilde
  foo.bak.1 foo.bak.2 bar.bak.3 baz~
  

• Collating elements: [.char.] matches the collating element char. A collating element is a character or sequence of characters treated as a single unit in pattern matching. Collating elements depend on the current locale and are not yet implemented in yash-rs.

• Equivalence classes: [=char=] matches the equivalence class of char. An equivalence class is a set of characters considered equivalent for matching purposes (e.g., a and A in some locales). This feature is not yet implemented in yash-rs.

Locale support is not yet implemented in yash-rs. Currently, all patterns match the same characters regardless of locale. Collating elements and equivalence classes simply match the characters as they are, without any special treatment.

Special considerations for pathname expansion

See the Pathname expansion section for additional rules that apply in pathname expansion.