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Patterns and "Globbing"

BASH offers three kinds of pattern matching.

  • glob
  • extended glob
  • regular expressions

Glob

Pattern Description
* Matches any string, of any length
foo* Matches any string beginning with foo
*foo* Matches any string containing foo (beginning, middle or end)
*.tar.gz Matches any string ending with .tar.gz
*.[ch] Matches any string ending with .c or .h but not .ch
foo? Matches food or foot but not foobar

When matching filenames, the * and ? characters will NOT match a slash (/) character.
When matching strings, the * and ? characters WILL match a slash (/) character.

With filename globbing, filenames are substituted after word splitting, all resulting filenames are literal and each filename is a separate word, even if filenames have spaces in them.

To disable filename globbing: set -f

Rules for brackets [ ]

If the first character following the [ is a ! or a ^ then any character NOT enclosed is matched. A literal - may be matched by including it as the first or last character in the set. A literal ] may be matched by including it as the first character in the set.

Character classes

Class Description Equivalent
[:alnum:] Alphanumeric (letters or digits) a-zA-Z0-9
[:alpha:] Alphabetic (upper and lower case letters) a-zA-Z
[:blank:] Space and tab \t
[:cntrl:] Control characters octal codes 000 through 037, and 177 (DEL)
[:digit:] Numbers in decimal 0-9
[:graph:] Printable characters excluding space [:alnum:] and [:punct:]
[:lower:] Lowercase letters a-z
[:print:] Printable characters including space [:alnum:] [:punct:] and space
[:punct:] Punctuation and symbols !"#$%&'() ...etc
[:space:] All whitespace characters \t\r\n\v\f
[:upper:] Uppercase letters A-Z
[:xdigit:] Hexadecimal digits A-Fa-f0-9

Note: In actual usage these character classes will be inside brackets, thus:
[[:alpha:]] (match a letter), but could also be [[:alpha:]7] (match a letter or 7)

globstar

Off by default. Enable with: shopt -s globstar

The standard * matches files and directories at the current level.
With globstar, ** matches zero or more directories.

Example: **/*.mp3 matches *.mp3 files recursively: in current directory, and all subdirectories below.

dotglob and nullglob

Off by default. Enable with: shopt -s dotglob nullglob

By default, * doesn't match hidden files.
Doing .* matches hidden files, but also matches the . and .. directory entries (this dir and the parent dir). Probably not what you want.

dotglob fixes this. But in an empty directory, this happens:

ls *
ls: cannot access '*': No such file or directory

Probably not what you want.
nullglob fixes this, so that ls * displays nothing in an empty directory.

Extended Glob

Off by default. Enable with: shopt -s extglob

?(pattern-list)  Matches ZERO or ONE occurrence of the given patterns
*(pattern-list)  Matches ZERO or MORE occurrences of the given patterns
+(pattern-list)  Matches ONE or MORE occurrences of the given patterns
@(pattern-list)  Matches exactly one of the given patterns
!(pattern-list)  Matches anything EXCEPT one of the given patterns

The list inside the parentheses is a list of globs or extended globs separated by the pipe | character.

Extended Glob Example

$ shopt -s extglob
$ ls
names.txt  tokyo.jpg  california.bmp
$ echo !(*jpg|*bmp)
names.txt

Regular Expressions

  • Regex can only be used with the Bash [[ keyword in Bash 3.0 or later.
  • cannot be used for filename matching.
  • The syntax is: [[ string =~ regex ]], usually used within an if...then...else statement
Condition exit code
string matches pattern 0 (true)
string does not match pattern 1 (false)
invalid pattern syntax shell aborts with exit code 2
  • Bash uses the Extended Regular Expression (ERE) dialect.
  • The BASH_REMATCH array variable holds the results of Regular Expressions that use capturing groups (parentheses).

The entire matched string: ${BASH_REMATCH[0]} First capture group: ${BASH_REMATCH[1]}

For best results:

  • put the regex in a variable, single quoted.
  • use this variable, unquoted in the [[ statement
  • if you quote the regex, it gets treated as a literal string.

Regular Expression Example

Here we capture the $LANG environment variable into 2 capture groups, and display the results.

# First, let's see what the $LANG environment variable is
echo $LANG
# Result:
en_US.UTF-8

# Put the regex in a variable, single quoted
regex='(..)_(..)'

# Use the regex, unquoted
if [[ $LANG =~ $regex ]]; then
    echo "Country code is ${BASH_REMATCH[2]}"
    echo "Language code is ${BASH_REMATCH[1]}"
else
    echo "Your locale is not recognized"
fi

# Result:
Country code is US
Language code is en