Espresso.jl is a new package for expression matching and transformation. A few functions that might be intersting for people working with Julia AST:

matchex
 match expression by pattern, extract matching elements; 
subs
 substitute elements of an expression according to substitution table; 
rewrite
 rewrite an expression by matching it to a pattern and replacing corresponding placeholders in substitution expression; 
simplify
 simplify algebraic expression.
For example, consider derivation rule:
f(x) = x^2
f'(x) = 2*x
One may rewrite existing expression to its derivative as easy as:
ex = :(u ^ 2) # expression to differentiate
pat = :(_x ^ 2) # _x is a placeholder  it will match actual variable
subsex = :(2 * _x) # substitution expression
rewrite(ex, pat, subsex) # gives :(2 * u)
Additionally, Espresso.jl supports extended Einstein notation suitable for describing nontrivial operations on tensors. For example, one can convert a vectorized expression to the Einstein notation as:
to_einstein(:(W*x + b); W=rand(3,4), x=rand(4), b=rand(3))
# quote
# tmp1[i] = W[i,k] * x[k]
# tmp2[i] = tmp1[i] + b[i]
# end
and backward as:
from_einstein(:(W[i,k] * x[k] + b[i]))
# quote
# tmp1 = W * x
# tmp2 = tmp1 + b
# end
Other supported functions may be found in README.md and the list of exports.