OAuth 2.0 spec RFC 6749 define that client_id should be in the format %x20-7E.

What this mean?

Let me explain using some Python code.

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>>> chr(int(0x20)), chr(int(0x7E))
(' ', '~')

But wait it is a range of characters.

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>>> characters = []
>>> for char in range(int(0x20), int(0x7E)):
...     characters.append(chr(char))
... 
>>> characters
[' ', '!', '"', '#', '$', '%', '&', "'", '(', ')', '*', '+', ',', '-', '.', '/', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', ':', ';', '<', '=', '>', '?', '@', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '[', '\\', ']', '^', '_', '`', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '{', '|', '}']
>>> ''.join(characters)
' !"#$%&\'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}'
>>>

So you can use the following code to create client_id for your RESTful API.

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>>> import random
>>> rand = random.SystemRandom()
>>> length=30
>>> ''.join(rand.choice(characters) for x in range(length))
'*?>tN7a00_FMu{DKMxI<!->&I4GuU7'
>>>

This code will create a client_id with 30 characters length.