PostgreSQL / PgRouting syntax error

My question is about Postgres/PgRouting syntax, when I wrote query like this:

SELECT * FROM shortest_path('SELECT gid  AS id,
      source::int4 AS source,
      target::int4 AS target,
      cost::float8 AS cost,
      reverse_cost::float8 AS reverse_cost
from network where country='USA'  ',                           
257027,
276521,
true,
true)

I got error:

ERROR:  syntax error at or near "USA"
LINE 6:  from network where country='USA'  ',             
ERROR: syntax error at or near "USA"
SQL state: 42601
Character: 226

So, I create new field in the network table countryInt and I updated field like this:

Update network set countryInt =1 where country='USA'

So my query is:

SELECT * FROM shortest_path('SELECT gid  AS id,
    source::int4 AS source,
    target::int4 AS target,
    cost::float8 AS cost,
    reverse_cost::float8 AS reverse_cost
from network where countryInt=1 ',                           
257027,
276521,
true,
true)

And now it works. But what if I have to put some string in condition, quotation marks are problem?

How to resolve this error?

Thank you in advance.