SQL Server's isNumeric() equivalent in amazon redshift

Question

• I\'m using amazon redshift as my data warehouse
• I have a field (field1)of type string. Some of the strings start with four numbers and others with letters:

Answer 1

where regexp_instr(field1,'^[0-9]{4}') = 0

will remove rows starting with 4 digits (the above regexp_instr will return 1 for the rows with field1 starting with 4 digits)