Bash regular expression match with groups including example to parse http_proxy environment variable

The newer versions of bash include a regex operator =~

Simple example

$ re='t(es)t'
$ [[ "test" =~ $re ]]
$ echo $?
0
$ echo ${BASH_REMATCH[1]}
es

Note

operator returns true if it's able to match
nested groups are possible (example below shows ordering)
- 0 is the whole match
- optional groups are counted even if not present and will be indexed, but be empty/null
global match isn't suported, so it only matches once
the regex must be provided as an unquoted variable reference to the re var

Match fails if re specified directly as string. Seems to want to be unquoted...

[[ "test" =~ 't(es)t' ]]; echo $?;
1

More complex example to parse the http_proxy env var

http_proxy_re='^https?://(([^:]{1,128}):([^@]{1,256})@)?([^:/]{1,255})(:([0-9]{1,5}))?/?'
if [[ "$http_proxy" =~ $http_proxy_re ]]; then
  # care taken to skip parent nesting groups 1 and 5
  user=${BASH_REMATCH[2]}
  pass=${BASH_REMATCH[3]}
  host=${BASH_REMATCH[4]}
  port=${BASH_REMATCH[6]}
fi

Examples of tricky issues with limitations:

Pattern backreference to an optional capturing subexpression
Multiple matches in a string using regex in bash

mcarbonneaux/bash_regex_match_groups.md