3 added 75 characters in body

# Any engine, except Apache, 11ECMAScript (11 bytes):

/^\1?d/
/\d/


Apache compatible (7 bytes):

# Other REGEXP Engines (14 bytes):

^\1/^\\\\1?d/
/\d/


The Apache version is just to "fill the blank", since it REQUIRES that there is no surrounding slashes. In no way this is a valid answer!

The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

EDIT:

Originally, this answer was posted as being compatible with all engines, but it was proven to be wrong.

There was a problem with references to the capturing groups (for example, in php).

# Any engine, except Apache, 11 bytes:

/^\1?d/
/\d/


Apache compatible (7 bytes):

^\1?d
\d


The Apache version is just to "fill the blank", since it REQUIRES that there is no surrounding slashes. In no way this is a valid answer!

The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

# ECMAScript (11 bytes):

/^\1?d/
/\d/


# Other REGEXP Engines (14 bytes):

/^\\\\1?d/
/\d/


The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

EDIT:

Originally, this answer was posted as being compatible with all engines, but it was proven to be wrong.

There was a problem with references to the capturing groups (for example, in php).

2 added 118 characters in body

# Any engine, except APACHEApache, 11 bytes:

/^\1?d/
/\d/


# Apache compatible (7 bytes):

Apache compatible (7 bytes):

^\1?d
\d


The Apache version is just to "fill the blank", since it REQUIRES that there is no surrounding slashes. In no way this is a valid answer!

The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

# Any engine, except APACHE, 11 bytes:

/^\1?d/
/\d/


# Apache compatible (7 bytes):

^\1?d
\d


The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

# Any engine, except Apache, 11 bytes:

/^\1?d/
/\d/


Apache compatible (7 bytes):

^\1?d
\d


The Apache version is just to "fill the blank", since it REQUIRES that there is no surrounding slashes. In no way this is a valid answer!

The 1st matches \d[..] or \1d[..].

The second matches any string with a number.

1

# Any engine, except APACHE, 11 bytes:

/^\1?d/
/\d/


# Apache compatible (7 bytes):

^\1?d
\d


The 1st matches \d[..] or \1d[..].

The second matches any string with a number.