Egrep special expressions like \w in bracket expressions [] - regex

Question

I am trying to use extended grep to extract data from a JSON. The regex I use is functional on my regexr instance, but for some reason it doesn't work in bash.
I tried many things, notably the bare double dash and various minor edits to the regex for escaping.
#!/bin/bash
networks='{ "networks": [ { "admin_state_up": true, "availability_zone_hints": [], "availability_zones": [], "created_at": "2019-03-12T23:45:13Z", "description": "", "id": "7188504a-72cb-4590-a9b0-414732017837", "ipv4_address_scope": null, "ipv6_address_scope": null, "is_default": false, "mtu": 1450, "name": "BLUE", "port_security_enabled": true, "project_id": "187d635aec4c43fe8e8918afb3a5c82e", "provider:network_type": "vxlan", "provider:physical_network": null, "provider:segmentation_id": 86, "revision_number": 2, "router:external": false, "shared": false, "status": "ACTIVE", "subnets": [], "tags": [], "tenant_id": "187d635aec4c43fe8e8918afb3a5c82e", "updated_at": "2019-03-12T23:45:13Z" }, { "admin_state_up": true, "availability_zone_hints": [], "availability_zones": [], "created_at": "2019-03-12T23:45:13Z", "description": "", "id": "ed82083f-0a7c-4322-a4fb-de8db23e2bae", "ipv4_address_scope": null, "ipv6_address_scope": null, "is_default": false, "mtu": 1450, "name": "RED", "port_security_enabled": true, "project_id": "187d635aec4c43fe8e8918afb3a5c82e", "provider:network_type": "vxlan", "provider:physical_network": null, "provider:segmentation_id": 108, "revision_number": 2, "router:external": false, "shared": false, "status": "ACTIVE", "subnets": [], "tags": [], "tenant_id": "187d635aec4c43fe8e8918afb3a5c82e", "updated_at": "2019-03-12T23:45:13Z" }, { "admin_state_up": true, "availability_zone_hints": [], "availability_zones": [], "created_at": "2019-03-12T23:45:13Z", "description": "", "id": "1eb6647e-869e-4e83-9468-43e2c320bccc", "ipv4_address_scope": null, "ipv6_address_scope": null, "is_default": false, "mtu": 1450, "name": "public", "port_security_enabled": true, "project_id": "187d635aec4c43fe8e8918afb3a5c82e", "provider:network_type": "vxlan", "provider:physical_network": null, "provider:segmentation_id": 32, "revision_number": 2, "router:external": false, "shared": false, "status": "ACTIVE", "subnets": [], "tags": [], "tenant_id": "187d635aec4c43fe8e8918afb3a5c82e", "updated_at": "2019-03-12T23:45:13Z" } ] }'
result=`echo $networks | grep -oE '"(id|name)": "([\w+-]+)"'`
echo $result
The aforementioned code doesn't work but if I switch to the following regex, it works. I just need to add extraction for id field too to be able to extract ids and names using \2 back reference (group 2)
grep -oE '"(id|name)": "(\w+)"'
Can you help me understand why the script doesn't work?
Full formatted JSON
{
"networks": [{
"admin_state_up": true,
"availability_zone_hints": [],
"availability_zones": [],
"created_at": "2019-03-12T23:45:13Z",
"description": "",
"id": "7188504a-72cb-4590-a9b0-414732017837",
"ipv4_address_scope": null,
"ipv6_address_scope": null,
"is_default": false,
"mtu": 1450,
"name": "BLUE",
"port_security_enabled": true,
"project_id": "187d635aec4c43fe8e8918afb3a5c82e",
"provider:network_type": "vxlan",
"provider:physical_network": null,
"provider:segmentation_id": 86,
"revision_number": 2,
"router:external": false,
"shared": false,
"status": "ACTIVE",
"subnets": [],
"tags": [],
"tenant_id": "187d635aec4c43fe8e8918afb3a5c82e",
"updated_at": "2019-03-12T23:45:13Z"
}, {
"admin_state_up": true,
"availability_zone_hints": [],
"availability_zones": [],
"created_at": "2019-03-12T23:45:13Z",
"description": "",
"id": "ed82083f-0a7c-4322-a4fb-de8db23e2bae",
"ipv4_address_scope": null,
"ipv6_address_scope": null,
"is_default": false,
"mtu": 1450,
"name": "RED",
"port_security_enabled": true,
"project_id": "187d635aec4c43fe8e8918afb3a5c82e",
"provider:network_type": "vxlan",
"provider:physical_network": null,
"provider:segmentation_id": 108,
"revision_number": 2,
"router:external": false,
"shared": false,
"status": "ACTIVE",
"subnets": [],
"tags": [],
"tenant_id": "187d635aec4c43fe8e8918afb3a5c82e",
"updated_at": "2019-03-12T23:45:13Z"
}, {
"admin_state_up": true,
"availability_zone_hints": [],
"availability_zones": [],
"created_at": "2019-03-12T23:45:13Z",
"description": "",
"id": "1eb6647e-869e-4e83-9468-43e2c320bccc",
"ipv4_address_scope": null,
"ipv6_address_scope": null,
"is_default": false,
"mtu": 1450,
"name": "public",
"port_security_enabled": true,
"project_id": "187d635aec4c43fe8e8918afb3a5c82e",
"provider:network_type": "vxlan",
"provider:physical_network": null,
"provider:segmentation_id": 32,
"revision_number": 2,
"router:external": false,
"shared": false,
"status": "ACTIVE",
"subnets": [],
"tags": [],
"tenant_id": "187d635aec4c43fe8e8918afb3a5c82e",
"updated_at": "2019-03-12T23:45:13Z"
}]
}

Answer 1

According to man grep:
The Backslash Character and Special Expressions
The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum:]]. ... A bracket expression is a list of characters enclosed by [ and ]. ... To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal - place it last.
Basically, \w is literally replaced by those characters when evaluated, giving you "([[[:alnum:]]+-]+)", which in a US standard locale gives you "([[a-zA-Z0-9]+-]+)".
Since a bracket expression is truncated by the first ] it sees (unless it is the first element of a bracket expression), the group is only [[[:alnum:]]+, or "1 or more of a digit, letter, and [. This expression is followed by -]+, meaning "exactly one hyphen and one or more ]". This is obviously pretty terrible.
If you try
echo $networks | grep -oE '"(id|name)": "([[:alnum:]+-]+)"'
I.e., \w without the outer bracket expression, the relevant part means "a group (surrounded by ") comprised of one or more digits, letters, hyphens, and plus signs", which outputs:
"id": "7188504a-72cb-4590-a9b0-414732017837"
"name": "BLUE"
"id": "ed82083f-0a7c-4322-a4fb-de8db23e2bae"
"name": "RED"
"id": "1eb6647e-869e-4e83-9468-43e2c320bccc"
"name": "public"

Answer 2

Using PERL (-P) instead of Extended (-E) regexp, looks like the \w is interpreted as expected, without escaping issue: note the -oP
result=$( echo $networks | grep -oP '"(id|name)": "([\w+-]+)"' ) ;
echo $result
"id": "7188504a-72cb-4590-a9b0-414732017837" "name": "BLUE" "id": "ed82083f-0a7c-4322-a4fb-de8db23e2bae" "name": "RED" "id": "1eb6647e-869e-4e83-9468-43e2c320bccc" "name": "public"

Answer 3

As a workaround (it does not resolve the "escaping \w issue)
result=$( echo $networks | grep -oE '"(id|name)": "([a-zA-Z_+-]+)"' ) ;
echo $result
Prints me:
"name": "BLUE" "name": "RED" "name": "public"
Note: prefer using $( ) syntax to execute sub shells rather than the backtick.