/
(?<![^\s>]) # LEFT boundary: preceding char is whitespace, '>' (end of a tag), or start-of-string
( # capture 1: the whole formula
(?![^<>]*>) # guard: don't start inside an HTML tag's attribute list
(?: # ============ HEAD — one or more element/bracket "units" ============
(?: # a "unit": an element/bracket run, then any trailing counts
(?:
(?:H[eogsf]?|L[iavru]|B[eahkri]?|C[arofmusenld]?|N[eiahopdb]?|O[sg]?|F[rlem]?|M[godtcn]|A[lrsgutmc]|S[icerngmb]?|P[uabotmrd]?|Kr?|T[icebmsalh]|V|Z[nr]|G[ade]|R[buhenagf]|Yb?|I[nr]?|Xe|E[urs]|D[ysb]|W|U) # any element symbol (all 118)
|[()\[\]] # ...or a bracket
)+
(?: # then any trailing count, in ANY encoding, a plain digit, or a repeat index:
(?:
[₁₂₃₄₅₆₇₈₉][₀₁₂₃₄₅₆₇₈₉]* # subscript glyphs H₂
|[¹²³⁴⁵⁶⁷⁸⁹][⁰¹²³⁴⁵⁶⁷⁸⁹]* # superscript glyphs x²
|\\u208[1-9](?:\\u208[0-9])* # \u escape, subscript H\u2082 (unparsed JSON)
|(?:\\u00[bB][239]|\\u207[4-9])(?:\\u2070|\\u00[bB][239]|\\u207[4-9])* # \u escape, superscript
|&\#(?:0*832[1-9]|[xX]0*208[1-9]);(?:&\#(?:0*832[0-9]|[xX]0*208[0-9]);)* # HTML entity, subscript H₂ / H₂
|&\#(?:0*(?:178|179|185|830[89]|831[0-3])|[xX]0*(?:[bB][239]|207[4-9]));(?:&\#(?:0*(?:178|179|185|8304|830[89]|831[0-3])|[xX]0*(?:2070|[bB][239]|207[4-9]));)* # HTML entity, superscript
|<su[bp]>[1-9][0-9]*<\/su[bp]> # <sub>/<sup> tag H<sub>2</sub>
)
|[1-9][0-9]* # ...or a plain inline integer
|[ₙₘₓ] # ...or a repeat-index glyph (…)ₙ ← NEW
|<su[bp]>[nmx]<\/su[bp]> # ...or a repeat index in a tag (…)<sub>n</sub> ← NEW
)*
)+ # one or more units (a lone valid element is allowed)
)
(?:[+-](?![A-Za-z0-9]))? # optional ionic charge on the head (K+, …F₃-)
(?: # ============ zero or more salt / hydrate components ============
(?:\s*[·•‧∙⋅・･*]\s*|\.(?=[A-Za-z(\[])) # separator: a spaced dot variant, OR a tight "." right before a component
(?: # optional coefficient: a sub/sup number, an integer/fraction, or x/n
[₁₂₃₄₅₆₇₈₉][₀₁₂₃₄₅₆₇₈₉]*|[¹²³⁴⁵⁶⁷⁸⁹][⁰¹²³⁴⁵⁶⁷⁸⁹]*|\\u208[1-9](?:\\u208[0-9])*|(?:\\u00[bB][239]|\\u207[4-9])(?:\\u2070|\\u00[bB][239]|\\u207[4-9])*|&\#(?:0*832[1-9]|[xX]0*208[1-9]);(?:&\#(?:0*832[0-9]|[xX]0*208[0-9]);)*|&\#(?:0*(?:178|179|185|830[89]|831[0-3])|[xX]0*(?:[bB][239]|207[4-9]));(?:&\#(?:0*(?:178|179|185|8304|830[89]|831[0-3])|[xX]0*(?:2070|[bB][239]|207[4-9]));)*|<su[bp]>[1-9][0-9]*<\/su[bp]>|[1-9][0-9]*(?:\/[1-9][0-9]*)?|[xn]
)?
(?: # the component's own units (elements/brackets + counts), one or more
(?:
(?:H[eogsf]?|L[iavru]|B[eahkri]?|C[arofmusenld]?|N[eiahopdb]?|O[sg]?|F[rlem]?|M[godtcn]|A[lrsgutmc]|S[icerngmb]?|P[uabotmrd]?|Kr?|T[icebmsalh]|V|Z[nr]|G[ade]|R[buhenagf]|Yb?|I[nr]?|Xe|E[urs]|D[ysb]|W|U)
|[()\[\]]
)+
(?:
(?:[₁₂₃₄₅₆₇₈₉][₀₁₂₃₄₅₆₇₈₉]*|[¹²³⁴⁵⁶⁷⁸⁹][⁰¹²³⁴⁵⁶⁷⁸⁹]*|\\u208[1-9](?:\\u208[0-9])*|(?:\\u00[bB][239]|\\u207[4-9])(?:\\u2070|\\u00[bB][239]|\\u207[4-9])*|&\#(?:0*832[1-9]|[xX]0*208[1-9]);(?:&\#(?:0*832[0-9]|[xX]0*208[0-9]);)*|&\#(?:0*(?:178|179|185|830[89]|831[0-3])|[xX]0*(?:[bB][239]|207[4-9]));(?:&\#(?:0*(?:178|179|185|8304|830[89]|831[0-3])|[xX]0*(?:2070|[bB][239]|207[4-9]));)*|<su[bp]>[1-9][0-9]*<\/su[bp]>)|[1-9][0-9]*|[ₙₘₓ]|<su[bp]>[nmx]<\/su[bp]> # ← NEW: repeat index also allowed on components
)*
)+
(?:[+-](?![A-Za-z0-9]))? # optional charge on the component
)*
)
(?![^\s<]) # RIGHT boundary: following char is whitespace, '<' (start of a tag), or end-of-string
/
gmx