Bash =~

Internal

• bash Concepts

Overview

The string to the right of the = ̃ operator is considered an extended regular expression and matched against the string to the left of the operator. The rules governing the extended regular expression are described in regex(3).

if [[ "string_to_be_matched_against_regex" =~ regex ]]; then
  # match 
else
  # no match 
fi

Usage is subject to the following constraints:

• The use of [[...]] is required.
• ⚠️DO NOT use double quotes when specifying the regular expression, just specify the regex literal directly. If you do double quote the right-hand side string, =~ will still work but it will match the quoted string literally rather than a regular expression.
• Variable expansion is performed, the following expression is valid:

if [["something" =~ ${regex} ]]; then ...

• If the regular expression contains spaces, use single quotes to denote space:

if [["something" =~ ${regex}' ' ]]; then ...

This also works:

[[ "${output}" =~ "chart type".*"required" ]]

• Variable substitution works for both the string to be matched and the regular expression. When using a variable to specify the regular expression, do not enclose the variable in quotes, use it directly as in the example below:

if [[ "${var_containing_string_to_be_matched_against_regex}" =~ ${var_containing_regex} ]]; then ...

• It matches [[:space:]]
• If any part of the pattern is quoted, it is forcibly matched as a string.
• The return value is 0 if the string matches the pattern, and 1 otherwise.
• If the regular expression is syntactically incorrect, the conditional expression's return value is 2.
• If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters.
• The =~ operator has the same precedence as == and !=.

Metacharacters

See man re_format.

Single digits:

[[:digit:]]

Examples

A string that does not contain a slash:

if [[ ${something} =~ ^[^/]*$ ]]; then
   ...
fi

Multiple spaces at the end:

if [[ ${something} =~ ^blah' '*$ ]]; then
   ...
fi

TODO

regex(3)