Plain text:

sed -i 's/[\d0]//g'  xxx.fasta

Gzipped:

gzip -cd xxx.fasta.gz | sed -i 's/[\d0]//g' | gzip -c > clean.fasta.gz

I read and save in biopython to remove all the trash. Something like:

less biopy_reformat_fastq_remove_short.py

## Colin, Feb. 2018
## Remove short length sequences reported in the fastq (intended for Pacbio downsampling)


from Bio import SeqIO
import sys

good_seqs=[]
c=0

if len(sys.argv) <= 2:
        print "Enter input and output files \neg. python biopy_reformat_fasta.py input.fa output.fa"
else:
        for record in SeqIO.parse(open(sys.argv[1], "rU"), "fastq"):
                # Change the minimum read length
                minimum_length = 3000
                if len(record.seq) >= minimum_length :
                        record = record.upper()
                        good_seqs.append(record)
                else:
                        c = c + 1


print "Found short sequences:" + str(c)

output = open(sys.argv[2], "w")
SeqIO.write(good_seqs, output, "fastq")
output.close()