Aligning Lines!

APL (37)

APL just isn't very good at string processing (or I'm not good at golfing, of course).

{⌽∊R,¨' '/⍨¨(⌈/-+)⍺⍳⍨¨⌽¨R←S⊂⍨S=⊃S←⌽⍵}

This takes the character as its left argument, and the multiline string as its right argument. It is assumed that the multiline string ends in a linefeed (e.g. A\nB\nC\n rather than A\nB\nC.) Since I can use "any format [I] wish", and this is also the conventional format for text files, I think this is reasonable.

Explanation:

• S←⌽⍵: reverse the string, and store it in S.
• R←S⊂⍨S=⊃S: split S on its first character, and store the array of strings in R.
• ⍺⍳¨⌽¨R: reverse each string in R, and then find the index of ⍺ (the character) in each string.
• (⌈/-+): subtract each of the indices from the largest index, giving the amount of spaces needed
• ' '/⍨¨: for each of those values, generate that many spaces
• R,¨: add the spaces to each string in R.
• ∊: join all the strings together
• ⌽: reverse it (to get the original order back)

Example:

      NL←⎕UCS 10 ⍝ newline
      test←'Programming, Puzzles',NL,'And, Code golf',NL
      test ⍝ test string
Programming, Puzzles                
And, Code golf                      

      ⍝ run the function
      +X←','{⌽∊R,¨' '/⍨¨(⌈/-+)⍺⍳⍨¨⌽¨R←S⊂⍨S=⊃S←⌽⍵}test
Programming, Puzzles                        
        And, Code golf                      

      ⍴X ⍝ result is really a string with newlines, not a matrix
44

CJam, 23 22 20 bytes

Thanks to Dennis for saving 2 bytes.

ea_rf#_:e>\fm.{S*\N}

This reads the lines from command-line arguments and the character from STDIN.

The online interpreter doesn't support command-line arguments, but you can test an equivalent version here.

Explanation

ea    e# Get the lines from ARGV.
_rf#  e# Duplicate input, read the character and find index of character in each line.
_:e>  e# Duplicate indices and find maximum.
\fm   e# Subtract each index from the maximum index.
.{    e# Apply this block to each pair of line and (max_index - index).
  S*  e#   Get a string with the right amount of spaces.
  \N  e#   Swap spaces with line and push a line feed.
}

Pip, 22 20 18 + 1 = 19 bytes

Y_@?qMgsX(MXy)-y.g

Takes strings as command-line arguments and delimiter from STDIN (idea borrowed from Martin's CJam answer). Uses -n flag to print output values on separate lines.

                    g is list of cmdline args; s is space (implicit)
    q               Read the delimiter from stdin
 _@?                Construct a lambda function that takes a string and returns
                       the index of the delimiter in it
     Mg             Map that function to each remaining item in g
Y                   Yank the resulting list of indices into the variable y

         (MXy)-y    Take the max of y minus each element in y
       sX           Space, repeated that many times...
                .g  ... concatenated to each item in g
                    Print, newline-separated (implicit, -n flag)

And an example run:

C:\Users\dlosc> pip.py -ne Y_@?qMgsX(MXy)-y.g "Programming, Puzzles" "And, Code golf"
,
Programming, Puzzles
        And, Code golf