1       2       3       4       c.2458C>T
a   b   c   d   c.45_46delAA
a1  b2  c3  d4  p.Ala78_Arg80del

gawk -F 't' -v OFS='t' 'match($5, /[[:digit:]]+/, m) {$(++NF) = m[0]} 1' file

1   2   3   4   c.2458C>T   6   2458
a   b   c   d   c.45_46delAA    or  f   45
a1  b2  c3  d4  p.Ala78_Arg80del    f6  78

perl -F't' -lane 'print join "t", @F, $F[4]=~/(d+)/' file

awk 'BEGIN {FS=OFS="t"} match($5,/[0-9]+/,arr) {print $0,arr[0]}' file

$ cat file
1       2       3       4       c.2458C>T       6
a       b       c       d       c.45_46delAA or f
a1      b2      c3      d4      p.Ala78_Arg80del        f6

$ sed -r 's/([^t]*t){4}[^[:digit:]]*([[:digit:]]+).*/2/' file
2458
45
78

sed -E 's/([^t]*t){4}[^[:digit:]]*([[:digit:]][[:digit:]]*).*/2/' file

$ awk '{sub(/^[^[:digit:]]*/, "", $5); sub(/[^[:digit:]].*/, "", $5); print $5;}' file
2458
45
78

sed 's/[^t0-9]*([0-9]*)[^t]*/1/5' <infile

xsel -bo | unexpand -a | sed ...

1   2   3   4   2458    6
a   b   c   d   45
a1  b2  c3  d4  78  f6

<infile
 cut -f5 | tr -cs '0-9n' \t |
 expand -t1,2,4 | cut -d' ' -f-2

2458
 45
 78

2458 45 78