Find: \h+
Replace: ,

I used \h+ to capture all spaces and replaced them with a single comma.

Result

First,String,Second,1.22,3.4
Second,More,Text,1.555555,2.2220
Third,x,3,124
Find: (\w+), (\w+), (.*)
Replace: \2 \1 (\3)

I used () and \w+ to capture target pieces of text and put them in desired order with \1, \2, and \3.

Result

Bryan Ballif (University of Vermont)
Aaron Ellison (Harvard Forest)
Sydne Record (Bryn Mawr)
Find:  \d
Replace: \n0

I used \d to capture the specific text piece space0 and replaced with \n0, for which I hard-coded in the number 0.

Result

0001 Georgia Horseshoe.mp3
0002 Billy In The Lowground.mp3
0003 Cherokee Shuffle.mp3
0004 Walking Cane.mp3
Find: (\d{4}) (.+).mp3
Replace: \2_\1.mp3

I used the wildcard \d with quantifier {4} to specify the first four digits and (.+) to extract the title, and re-ordered them with \2_\1.

Result

Georgia Horseshoe_0001.mp3
Billy In The Lowground_0002.mp3
Cherokee Shuffle_0003.mp3
Walking Cane_0004.mp3
Find: (\w)\w+,(\w+).*,(\d*)
Replace: \1_\2,\3

Not a neat regular expression definitely, but I hard-coded in the format and used wildcards with () to capture the desired pieces of text and put them in the new format.

Result

C_pennsylvanicus,44
C_herculeanus,3
M_punctiventris,4
L_neoniger,55
Find: (\w)\w+,(\w{4}).*,(\d*)
Replace: \1_\2,\3

I changed the quantifier in the second () to {4} to just extract the four letters of species name.

Result

C_penn,44
C_herc,3
M_punc,4
L_neon,55
Find: (\w{3})\w+,(\w{3})\w+,(.*),(\d*)
Replace: \1\2, \4, \3

Still, I designed the find and replace terms to match the desired formats while used () and \1,\2,\3,\4 to capture target pieces of text and re-order them.

Result

Campen, 44, 10.2
Camher, 3, 10.5
Myrpun, 4, 12.2
Lasneo, 55, 3.3