parson.sh

#!/bin/bash

parse_pid() {
  local IFS=$'\n'

  s_regex="^ $1:.*"
  w_regex="^   $2:.*"
  p_regex="^     $3:.*"
  n_regex='^ [^ ]' # we've gone too far!

  first=true
  matching_pane=false

  # assumption: no more than 100 lines to parse per session
  grep -A 100 "$s_regex" | while read -r line; do
    if $matching_pane; then
      if [[ "$line" =~ $p_regex ]]; then
        awk '{print $3}' <<< $line
        break
      fi
    else
      if [[ "$line" =~ $w_regex ]]; then
        matching_pane=true;
        continue
      fi
    fi

    # we hit next session with no match
    if ! $first && [[ "$line" =~ $n_regex ]]; then
      echo 'no match' >&2
      break
    fi

    first=false
  done
}

# outputs 19131 when given http://paste.pocoo.org/raw/336726/ (minus markup)
parse_pid 0 4 0 < './test'