[R] regular expression for selection

[Petr PIKAL]

Dear all

I am again (as usual) lost in regular expression use for selection. Here 
are my data:

> dput(mena)
c("138516_10g_50ml_50c_250utes1_m53.00-_s1.imp", 
"138516_10g_50ml_50c_250utes1_m54.00_s1.imp", 
"138516_10g_50ml_50c_250utes1_m55.00_s1.imp", 
"138516_10g_50ml_50c_250utes1_m56.00_s1.imp", 
"138516_10g_50ml_50c_250utes1_m57.00_s1.imp", 
"138516_10g_50ml_50c_250utes1_m58.00_s1.imp", 
"138516_10g_50ml_50c_250utes1_m59.00_s1.imp")

I want to select only values "m" foolowed by numbers from 53 to 59.

I used

sub("m5.", "", mena)

which correctly selects those m53 - m59 values but, in contrary to my 
expectation, it replaced the selected values with specified replacement - 
in that case empty string. 

What I shall use if I want to get rid of all but m53-m59 from those 
strings?

Regards
Petr