Regex pattern for splitting BEM string into parts (PHP) - php

Question

I would like to isolate the block, element and modifier parts of a string via PHP regex. The flavour of BEM I'm using is lowercase and hyphenated. For example:
this-defines-a-block__this-defines-an-element--this-defines-a-modifier
My string is always formatted as the above, so the regex does not need to filter out any invalid BEM, for example, I will never have dirty strings such as:
This.defines-a-block__this-Defines-an-ELEMENT--090283
Block, Element and Modifier names could contain numbers, so we could have any combination of the following:
this-is-block-001__this-is-element-001--modifier-002
Finally a modifier is optional so not every string will have one for example:
this-is-a-block-001__this-is-an-element
this-is-a-block-002__this-is-an-element--this-is-an-optional-modifier
I am looking for some regex to return each section of the BEM markup. Each string will be isolated and sent to the regex individually, not as a group or as multiline strings. The following sent individually:
# String 1
block__element--modifier
# String 2
block-one__element-one--modifier-one
# String 3
block-one-big__element-one-big--modifier-one-big
# String 4
block-one-001__element-one-001
Would return:
# String 1
block
element
modifier
# String 2
block-one
element-one
modifier-one
# String 3
block-one-big
element-one-big
modifier-one-big
# String 4
block-one-001
element-one-001

Answer 1

You could use 3 capturing groups and make the third one optional using the ?
As all 3 groups are lowercase, can contain numbers and use the hyphen as a delimiter you might use a character class [a-z0-9].
You could reuse the pattern for group 1 using (?1)
\b([a-z0-9]+(?:-[a-z0-9]+)*)__((?1))(?:--((?1)))?\b
Explanation
\b Word boundary
( First capturing group
[a-z0-9]+ Repeat 1+ times what is listed in the character class
(?:-[a-z0-9]+)* Repeat 0+ times matching - and 1+ times what is in the character class
) Close group 1
__ Match literally
((?1)) Capturing group 2, recurse group 1
(?: Non capturing group
-- Match literally
((?1)) Capture group 3, recurse group 1
)? Close non capturing group and make it optional
\b Word boundary
Regex demo
Or using named groups:
\b(?<block>[a-z0-9]+(?:-[a-z0-9]+)*)__(?<element>(?&block))(?:--(?<modifier>(?&block)))?\b
Regex demo