# Pattern matching and regular expressions

# Check if a string matches a regular expression

Check if a string consists in exactly 8 digits:

$ date=20150624
$ [[ $date =~ ^[0-9]{8}$ ]] && echo "yes" || echo "no"
yes
$ date=hello
$ [[ $date =~ ^[0-9]{8}$ ]] && echo "yes" || echo "no"
no

# Behaviour when a glob does not match anything

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

In case the glob does not match anything the result is determined by the options nullglob and failglob. If neither of them are set, Bash will return the glob itself if nothing is matched

$ echo no*match
no*match

If nullglob is activated then nothing (null) is returned:

$ shopt -s nullglob
$ echo no*match

$

If failglob is activated then an error message is returned:

$ shopt -s failglob
$ echo no*match
bash: no match: no*match
$

Notice, that the failglob option supersedes the nullglob option, i.e., if nullglob and failglob are both set, then - in case of no match - an error is returned.

# Get captured groups from a regex match against a string

a='I am a simple string with digits 1234'
pat='(.*) ([0-9]+)'
[[ "$a" =~ $pat ]]
echo "${BASH_REMATCH[0]}"
echo "${BASH_REMATCH[1]}"
echo "${BASH_REMATCH[2]}"

Output:

I am a simple string with digits 1234
I am a simple string with digits
1234

# The * glob

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

The asterisk * is probably the most commonly used glob. It simply matches any String

$ echo *acy
macy stacy tracy

A single * will not match files and folders that reside in subfolders

$ echo *
emptyfolder folder macy stacy tracy
$ echo folder/*
folder/anotherfolder folder/subfolder

# The ** glob

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -s globstar

Bash is able to interpret two adjacent asterisks as a single glob. With the globstar option activated this can be used to match folders that reside deeper in the directory structure

echo **
emptyfolder folder folder/anotherfolder folder/anotherfolder/content folder/anotherfolder/content/deepfolder folder/anotherfolder/content/deepfolder/file folder/subfolder folder/subfolder/content folder/subfolder/content/deepfolder folder/subfolder/content/deepfolder/file macy stacy tracy

The ** can be thought of a path expansion, no matter how deep the path is. This example matches any file or folder that starts with deep, regardless of how deep it is nested:

$ echo **/deep*
folder/anotherfolder/content/deepfolder folder/subfolder/content/deepfolder

# The ? glob

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

The ? simply matches exactly one character

$ echo ?acy
macy
$ echo ??acy
stacy tracy

# The [ ] glob

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

If there is a need to match specific characters then '[]' can be used. Any character inside '[]' will be matched exactly once.

$ echo [m]acy
macy
$ echo [st][tr]acy
stacy tracy

The [] glob, however, is more versatile than just that. It also allows for a negative match and even matching ranges of characters and characterclasses. A negative match is achieved by using ! or ^ as the first character following [. We can match stacy by

$ echo [!t][^r]acy
stacy

Here we are telling bash the we want to match only files which do not not start with a t and the second letter is not an r and the file ends in acy.

Ranges can be matched by seperating a pair of characters with a hyphen (-). Any character that falls between those two enclosing characters - inclusive - will be matched. E.g., [r-t] is equivalent to [rst]

$ echo [r-t][r-t]acy
stacy tracy

Character classes can be matched by [:class:], e.g., in order to match files that contain a whitespace

$ echo *[[:blank:]]*
file with space

# Matching hidden files

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

The Bash built-in option dotglob allows to match hidden files and folders, i.e., files and folders that start with a .

$ shopt -s dotglob
$ echo *
file with space folder .hiddenfile macy stacy tracy

# Case insensitive matching

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

Setting the option nocaseglob will match the glob in a case insensitive manner

$ echo M*
M*
$ shopt -s nocaseglob
$ echo M*
macy

# Extended globbing

Preparation

$ mkdir globbing
$ cd globbing
$ mkdir -p folder/{sub,another}folder/content/deepfolder/
touch macy stacy tracy "file with space" folder/{sub,another}folder/content/deepfolder/file .hiddenfile
$ shopt -u nullglob
$ shopt -u failglob
$ shopt -u dotglob
$ shopt -u nocaseglob
$ shopt -u extglob
$ shopt -u globstar

Bash's built-in extglob option can extend a glob's matching capabilities

shopt -s extglob

The following sub-patterns comprise valid extended globs:

?(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 one of the given patterns
!(pattern-list) – Matches anything except one of the given patterns

The pattern-list is a list of globs separated by |.

$ echo *([r-t])acy
stacy tracy

$ echo *([r-t]|m)acy
macy stacy tracy

$ echo ?([a-z])acy
macy

The pattern-list itself can be another, nested extended glob. In the above example we have seen that we can match tracy and stacy with *(r-t). This extended glob itself can be used inside the negated extended glob !(pattern-list) in order to match macy

$ echo !(*([r-t]))acy
macy

It matches anything that does not start with zero or more occurrences of the letters r, s and t, which leaves only macy as possible match.

# Regex matching

pat='[^0-9]+([0-9]+)'
s='I am a string with some digits 1024'
[[ $s =~ $pat ]] # $pat must be unquoted
echo "${BASH_REMATCH[0]}"
echo "${BASH_REMATCH[1]}"

Output:

I am a string with some digits 1024
1024

Instead of assigning the regex to a variable ($pat) we could also do:

[[ $s =~ [^0-9]+([0-9]+) ]]

Explanation

The [[ $s =~ $pat ]] construct performs the regex matching
The captured groups i.e the match results are available in an array named BASH_REMATCH
The 0th index in the BASH_REMATCH array is the total match
The i'th index in the BASH_REMATCH array is the i'th captured group, where i = 1, 2, 3 ...

# Syntax

$ shopt -u option # Deactivate Bash's built-in 'option'
$ shopt -s option # Activate Bash's built-in 'option'

# Remarks

Character Classes

Valid character classes for the [] glob are defined by the POSIX standard:

alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit

Inside [] more than one character class or range can be used, e.g.,

$ echo a[a-z[:blank:]0-9]*

will match any file that starts with an a and is followed by either a lowercase letter or a blank or a digit.

It should be kept in mind, though, that a [] glob can only be wholly negated and not only parts of it. The negating character must be the first character following the opening [, e.g., this expression matches all files that do not start with an a

$ echo [^a]*

The following does match all files that start with either a digit or a ^

$ echo [[:alpha:]^a]*

It does not match any file or folder that starts with with letter except an a because the ^ is interpreted as a literal ^.

Escaping glob characters

It is possible that a file or folder contains a glob character as part of its name. In this case a glob can be escaped with a preceding \ in order for a literal match. Another approach is to use double "" or single '' quotes to address the file. Bash does not process globs that are enclosed within "" or ''.

Difference to Regular Expressions

The most significant difference between globs and Regular Expressions is that a valid Regular Expressions requires a qualifier as well as a quantifier. A qualifier identifies what to match and a quantifier tells how often to match the qualifier. The equivalent RegEx to the * glob is .* where . stands for any character and * stands for zero or more matches of the previous character. The equivalent RegEx for the ? glob is .{1}. As before, the qualifier . matches any character and the {1} indicates to match the preceding qualifier exactly once. This should not be confused with the ? quantifier, which matches zero or once in a RegEx. The [] glob is can be used just the same in a RegEx, as long as it is followed by a mandatory quantifier.

Equivalent Regular Expressions

Glob	RegEx
`*`	`.*`
`?`	`.`
`[]`	`[]`

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