Using concat() function, is specific cases [closed] - postgresql

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I need concatenate regexp pattern pieces, for this pattern, I use C-style Escape E.
If use concatenation operator ||, works:
E'a{'||2||'}'
does not make much sense, but just interes, how to concatenate same, using concat() function ?

Answer 1

The misunderstanding is this: C-style escapes are just another way to input string literals. When you concatenate strings, be it with the || operator or with the concat() function (Postgres 9.1+), the method how individual strings were input is irrelevant.
In addition to that, literals of other types (like the numeric constant 2 in your example) are coerced to text automatically.
On top of that, your example does not exhibit any characters with a special meaning in escape strings (like \).
SELECT E'a{' || 2 || '}';
SELECT concat(E'a{', 2, '}');
So, the E is totally irrelevant in this particular example.
Since you mention regexp patterns: those tend to have \ in them, which have to be escaped with \ in E'' notation:
SELECT E'\\.' || 2 || '\.';
The modern way is not to use escape strings at all if not necessary. That's why Postgres switched to standard_conforming_strings = ON with PostgreSQL 9.1. That is the setting I tested with.