Did you know ... Search Documentation:
Predicate sub_atom/5
Availability:built-in
[ISO]sub_atom(+Atom, ?Before, ?Length, ?After, ?SubAtom)
ISO predicate for breaking atoms. It maintains the following relation: SubAtom is a sub-atom of Atom that starts at (0-based index) Before, has Length characters, and Atom contains After characters after the match. The implementation minimises non-determinism and creation of atoms. This is a flexible predicate that can do search, prefix- and suffix-matching, etc. Scenarios that use this predicate often generate atoms that with a short lifetime; in such cases sub_string/5 may be a better alternative. Examples:

Pick out a sub-atom of length 3 starting a 0-based index 2: 

?- sub_atom(aaxyzbbb, 2, 3, After, SubAtom).
After = 3,
SubAtom = xyz.

The following example splits a string of the form <name>=<value> into the name part (an atom) and the value (a string). 

name_value(String, Name, Value) :-
    sub_atom(String, Before, _, After, "="),
    !,
    sub_atom(String, 0, Before, _, Name),
    sub_atom(String, _, After, 0, Value).

This example defines a predicate that inserts a value at a position. Note that sub_string/5 is used here instead of sub_atom/5 to avoid the overhead of creating atoms for the intermediate results. 

atom_insert(Str, Val, At, NewStr) :-
    sub_string(Str, 0, At, A1, S1),
    sub_string(Str, At, A1, _, S2),
    atomic_list_concat([S1,Val,S2], NewStr).

On backtracking, matches are delivered in order left-to-right (i.e. Before increases monotonically): 

?- sub_atom('xATGATGAxATGAxATGAx', Before, Length, After, 'ATGA').
Before = 1, Length = 4, After = 14 ;
Before = Length, Length = 4, After = 11 ;
Before = 9, Length = 4, After = 6 ;
Before = 14, Length = 4, After = 1 ;
false.

See also sub_string/5, the corresponding predicate for SWI-Prolog strings.

Examples

Search for sub-atoms

Search for sub-atoms

To enumerate the 0-based starting indices of a possibly overlapping sub-atom:

?- sub_atom(banana, Pos, _, _, ana).
Pos = 1 ;
Pos = 3 ;
false.
Tags are associated to your profile if you are logged in
Tags:
Paul Singleton said (2013-08-05T18:33:39):1 upvotes 1 0 downvotes
Picture of user Paul Singleton.
Please guarantee to generate (+,-,-,-,+) solutions left-most match 1st etc. (as it seems to do)
RdR said (2020-03-20T04:00:39):0 upvotes 0 0 downvotes
Picture of user RdR.

Another example

% split username from email account
email_user(Email,User):-
   sub_atom(Email,U,1,A,'@'),
   T is A+1,
   sub_atom(Email,U,T,_,Tail),
   atom_concat(User,Tail,Email),!.

:- email_user('john@mail.com',User)
U = 'john'
Peter Ludemann said (2019-10-11T21:29:28):0 upvotes 0 0 downvotes
Picture of user Peter Ludemann.

check that suffix is '.bar': sub_atom('foo.bar', _, _, 0, '.bar').

faster version of removing a suffix suffix (same as atom_concat(Start, Suffix, Atom) if Suffix is ground:

sub_atom(Atom, Before, _, 0, Suffix),
Before0 is Before - 1,
sub_atom(Atom, 0, Before0, _, Start)
Peter Ludemann said (2019-11-13T18:07:45):0 upvotes 0 0 downvotes
Picture of user Peter Ludemann.

Strip off a prefix, e.g., '/' (from an absolute file name):

?- Str='/abc/def',
   sub_atom(Str, 0, _, After, '/'),
   sub_atom(Str, 1, After, 0, Sub).
Str = '/abc/def',
After = 7,
Sub = 'abc/def'.

This is equivalent to:

?- Str='/abc/def',
   atom_concat('/', Sub, Str).
Str = '/abc/def',
Sub = 'abc/def'.
Andrae Muys said (2013-10-22T01:58:20):0 upvotes 0 0 downvotes
Picture of user Andrae Muys.

A useful example is:

?- sub_atom('123456789', Before, Len, After, '3').
Before = 2,
Len = 1,
After = 6 .