When you use a collective variable that has many calculated components and you want to refer to them as arguments you can use regular expressions.

If your machine enables this (i.e. has "regex.h" C header ) then you can compile plumed with the preprocessor flag -D__PLUMED_HAS_CREGEX

Regular expressions are enclosed in round braces and must not contain spaces (the components name have no spaces indeed, so why using them?).

As an example

d1: DISTANCE ATOMS=1,2 COMPONENTS
PRINT ARG=(d1\.[xy])   STRIDE=100 FILE=colvar FMT=%8.4f

selects for printout both d1.x and d1.y components which are generated through distance in the order they are created by plumed. Notice that the "." character should be escaped if one wants to interpret it as a literal ".". Unescaped dots will match any character, according to regular expression syntax. E.g.

d1: DISTANCE ATOMS=1,2 COMPONENTS
dxy: DISTANCE ATOMS=1,3

# this will match d1.x,d1.y,dxy
PRINT ARG=(d1.[xy])   STRIDE=100 FILE=colvar FMT=%8.4f

# this will match d1.x,d1.y only
PRINT ARG=(d1\.[xy])   STRIDE=100 FILE=colvar FMT=%8.4f

You can also include more by using comma separated regular expressions

t1: TORSION ATOMS=5,7,9,15
t2: TORSION ATOMS=7,9,15,17
d1: DISTANCE ATOMS=7,17 COMPONENTS
PRINT ARG=(d1\.[xy]),(t[0-9]) STRIDE=100 FILE=colvar FMT=%8.4f

(this selects t1,t2,d1.x and d2.x) Beware if you have overlapping selection they will be duplicated.

You can equivalently use (recommended) regexp themself with the "or" operator.

t1: TORSION ATOMS=5,7,9,15
t2: TORSION ATOMS=7,9,15,17
d1: DISTANCE ATOMS=7,17 COMPONENTS
PRINT ARG=(d1\.[xy]|t[0-9]) STRIDE=100 FILE=colvar FMT=%8.4f

that selects the same arguments.

In the log produced you can see if your regular expression is effectively matching the component you want.

For more info see, for example, http://www.regular-expressions.info/reference.html.