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

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

• 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 ...

• 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 !=.