XPath Completion:

/a/b/c[e/f/g[foo((h|i[z]),/g)]]

NOTE: \& denotes the match
      \1..\n denotes n'th matched bracked

parse reversed string:

SPACE:    matches whitespace
NAME:     matches [-_[:alnum:]][-:_[:alnum:]]*[_[:alpha:]]
NAMECHAR: matches [-_[:alnum:]]
NNAMECHAR: matches [-:_[:alnum:]]


OPER:  matches ',' | '=' | '<' | '>' | '+' | '|' |
               '-' (?!NAME) |
               ('vid'|'dom'|'dna'|'ro') (SPACE* ']' | SPACE* ')' | SPACE* [0-9]+(?!NNAMECHAR) | SPACE NAMECHAR | SPACE '*')               
               '*' SPACE* ('*' | NAME | ')' | ']' | '.' )) |

WILDCARD: '*' (?!'*' | NAME | ')' | ']' | '.' ))
           


0. if SPACE * ']' -> [return: '']

1. if (NAMECHAR+) ':' (NNAMECHARS) '@' 
      store @\2:*[starts-with(local-name(),"\1")]
   if (NAMECHAR+) '@'
      store @*[starts-with(local-name(),"\1")]
   if (NAMECHAR+) ':' (NNAMECHAR+) 
      store \2:*[starts-with(local-name(),"\1")]
   if (NAMECHAR+)
      store *[starts-with(local-name(),"\1")]
   if '@'
      store '@*'
   else
      store '*'
   -> [2]

2. store while ':' | '::' | '@' | '/' | NAME '$' | NAME | WILDCARD | ballanced '[]' and '()' 
   -> [3]

3. if last = '/'
      if "'" or '"' -> [return: '']    # unballanced
      if SPACE* '(' | '[' | OPER -> [end]
      else -> [end] # uncertain if / with spaces are used
   else
      if SPACE+ (?! OPER ) -> [end]

  -> [4]

4. skip (ballanced(']', ')', '"', "'"), anychar) till '[' or end
   if '[' 
     store '/' -> [2]
   if end -> [end]

Example:

/a/b[2]/c[vvv + e/f/g[foo((h|i[z<TAB> -> /a/b[2]/c/e/f/g/i/starts-with(.,"z")