INTERNAL_NUM=$(grep -E 's*internal_nums*=' file.xml |sed -E 's/internal_nums*=s*([0-9]+)/1/')
echo "$INTERNAL_NUM"

internal_num = 1234

grep -oP 'binternal_nums*=s*Kd+' file.xml

$ echo '
      <root>
          <tag>
              <tag>
                  <wanted internal_num="1234" />
                  <wanted internal_num = "5678" />
              </tag>
          </tag>
      </root>
  ' | xmlstarlet sel -t -v '//@internal_num' -n

1234
5678

INTERNAL_NUM=$(grep -E 's*internal_nums*=' file.xml |sed -E 's/internal_nums*=s*([0-9]+)/1/')
echo "$INTERNAL_NUM"

$ echo "internal_num = 1234" | sed -E 's/internal_nums*=s*([0-9]+)/1/'
internal_num = 1234

$ echo "internal_num = 1234" | sed -E 's/internal_nums*=s*([0-9]+)/1/'
1234

awk '$1 == "internal_num"{print $NF; exit}' file

sed 's/internal_num[[:space:]]=[[:space:]]([0-9]*)/1/' file

s=[[:space:]]
sed -n "s/^(.*$s){0,1}internal_num$s*=$s*([0-9]{1,}).*/2/p"

grep -Po '(?<!S)internal_nums*=s*Kd+'