diff --git a/Cargo.toml b/Cargo.toml
index a43dcd83b..d976558a0 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,6 +21,7 @@ all = [
     "echo",
     "env",
     "expand",
+    "expr",
     "factor",
     "false",
     "fmt",
@@ -96,6 +97,7 @@ du       = { optional=true, path="src/du" }
 echo     = { optional=true, path="src/echo" }
 env      = { optional=true, path="src/env" }
 expand   = { optional=true, path="src/expand" }
+expr     = { optional=true, path="src/expr" }
 factor   = { optional=true, path="src/factor" }
 false    = { optional=true, path="src/false" }
 fmt      = { optional=true, path="src/fmt" }
diff --git a/Makefile b/Makefile
index 6d2f08a8e..061a1476d 100644
--- a/Makefile
+++ b/Makefile
@@ -53,6 +53,7 @@ PROGS       := \
   echo \
   env \
   expand \
+  expr \
   factor \
   false \
   fmt \
@@ -144,7 +145,7 @@ INSTALLEES  := \
 
 # Shared library extension
 SYSTEM := $(shell uname)
-DYLIB_EXT := 
+DYLIB_EXT :=
 ifeq ($(SYSTEM),Linux)
 	DYLIB_EXT    := so
 	DYLIB_FLAGS  := -shared
diff --git a/src/expr/Cargo.toml b/src/expr/Cargo.toml
new file mode 100644
index 000000000..164c2070d
--- /dev/null
+++ b/src/expr/Cargo.toml
@@ -0,0 +1,13 @@
+
+[package]
+name = "expr"
+version = "0.0.1"
+authors = []
+
+[lib]
+name = "expr"
+path = "expr.rs"
+
+[dependencies]
+getopts = "*"
+libc = "*"
diff --git a/src/expr/expr.rs b/src/expr/expr.rs
new file mode 100644
index 000000000..d6a7664d2
--- /dev/null
+++ b/src/expr/expr.rs
@@ -0,0 +1,135 @@
+#![crate_name = "expr"]
+
+/*
+ * This file is part of the uutils coreutils package.
+ *
+ * (c) Roman Gafiyatullin <r.gafiyatullin@me.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+extern crate getopts;
+extern crate libc;
+
+#[path="../common/util.rs"]
+#[macro_use]
+mod util;
+mod tokens;
+mod syntax_tree;
+
+use std::io::{Write};
+
+static NAME: &'static str = "expr";
+static VERSION: &'static str = "0.0.1";
+
+pub fn uumain(args: Vec<String>) -> i32 {
+	// For expr utility we do not want getopts.
+	// The following usage should work without escaping hyphens: `expr -15 = 1 +  2 \* \( 3 - -4 \)`
+
+	if maybe_handle_help_or_version( &args ) { 0 }
+	else {
+		let token_strings = args[1..].to_vec();
+
+		match process_expr( &token_strings ) {
+			Ok( expr_result ) => print_expr_ok( &expr_result ),
+			Err( expr_error ) => print_expr_error( &expr_error )
+		}
+	}
+}
+
+fn process_expr( token_strings: &Vec<String> ) -> Result< String, String > {
+	let maybe_tokens = tokens::strings_to_tokens( &token_strings );
+	let maybe_ast = syntax_tree::tokens_to_ast( maybe_tokens );
+	let maybe_result = evaluate_ast( maybe_ast );
+
+	maybe_result
+}
+
+fn print_expr_ok( expr_result: &String ) -> i32 {
+	println!("{}", expr_result);
+	if expr_result == "0" || expr_result == "" { 1 }
+	else { 0 }
+}
+
+fn print_expr_error( expr_error: &String ) -> ! {
+	crash!(2, "{}", expr_error)
+}
+
+fn evaluate_ast( maybe_ast: Result<Box<syntax_tree::ASTNode>, String> ) -> Result<String, String> {
+	if maybe_ast.is_err() { Err( maybe_ast.err().unwrap() ) }
+	else { maybe_ast.ok().unwrap().evaluate() }
+}
+
+
+
+fn maybe_handle_help_or_version( args: &Vec<String> ) -> bool {
+	if args.len() == 2 {
+		if args[1] == "--help" { print_help(); true }
+		else if args[1] == "--version" { print_version(); true }
+		else { false }
+	}
+	else { false }
+}
+
+fn print_help() {
+	//! The following is taken from GNU coreutils' "expr --help" output.
+	print!(
+r#"Usage: expr EXPRESSION
+  or:  expr OPTION
+
+      --help       display this help and exit
+      --version    output version information and exit
+
+Print the value of EXPRESSION to standard output.  A blank line below
+separates increasing precedence groups.  EXPRESSION may be:
+
+  ARG1 | ARG2       ARG1 if it is neither null nor 0, otherwise ARG2
+
+  ARG1 & ARG2       ARG1 if neither argument is null or 0, otherwise 0
+
+  ARG1 < ARG2       ARG1 is less than ARG2
+  ARG1 <= ARG2      ARG1 is less than or equal to ARG2
+  ARG1 = ARG2       ARG1 is equal to ARG2
+  ARG1 != ARG2      ARG1 is unequal to ARG2
+  ARG1 >= ARG2      ARG1 is greater than or equal to ARG2
+  ARG1 > ARG2       ARG1 is greater than ARG2
+
+  ARG1 + ARG2       arithmetic sum of ARG1 and ARG2
+  ARG1 - ARG2       arithmetic difference of ARG1 and ARG2
+
+  ARG1 * ARG2       arithmetic product of ARG1 and ARG2
+  ARG1 / ARG2       arithmetic quotient of ARG1 divided by ARG2
+  ARG1 % ARG2       arithmetic remainder of ARG1 divided by ARG2
+
+  STRING : REGEXP   [NOT IMPLEMENTED] anchored pattern match of REGEXP in STRING
+
+  match STRING REGEXP        [NOT IMPLEMENTED] same as STRING : REGEXP
+  substr STRING POS LENGTH   substring of STRING, POS counted from 1
+  index STRING CHARS         index in STRING where any CHARS is found, or 0
+  length STRING              length of STRING
+  + TOKEN                    interpret TOKEN as a string, even if it is a
+                               keyword like 'match' or an operator like '/'
+
+  ( EXPRESSION )             value of EXPRESSION
+
+Beware that many operators need to be escaped or quoted for shells.
+Comparisons are arithmetic if both ARGs are numbers, else lexicographical.
+Pattern matches return the string matched between \( and \) or null; if
+\( and \) are not used, they return the number of characters matched or 0.
+
+Exit status is 0 if EXPRESSION is neither null nor 0, 1 if EXPRESSION is null
+or 0, 2 if EXPRESSION is syntactically invalid, and 3 if an error occurred.
+
+Environment variables:
+	* EXPR_DEBUG_TOKENS=1   dump expression's tokens
+	* EXPR_DEBUG_RPN=1      dump expression represented in reverse polish notation
+	* EXPR_DEBUG_SYA_STEP=1 dump each parser step
+	* EXPR_DEBUG_AST=1      dump expression represented abstract syntax tree
+"#
+	);
+}
+
+fn print_version() {
+	println!("{} {}", NAME, VERSION);
+}
diff --git a/src/expr/syntax_tree.rs b/src/expr/syntax_tree.rs
new file mode 100644
index 000000000..53819974d
--- /dev/null
+++ b/src/expr/syntax_tree.rs
@@ -0,0 +1,386 @@
+/*
+ * This file is part of the uutils coreutils package.
+ *
+ * (c) Roman Gafiyatullin <r.gafiyatullin@me.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+//!
+//! Here we employ shunting-yard algorithm for building AST from tokens according to operators' precedence and associativeness.
+//! * https://en.wikipedia.org/wiki/Shunting-yard_algorithm
+//!
+
+use tokens::{Token};
+
+type TokenStack = Vec<(usize, Token)>;
+type OperandsList = Vec< Box<ASTNode> >;
+
+#[derive(Debug)]
+pub enum ASTNode {
+    Leaf { token_idx: usize, value: String },
+    Node { token_idx: usize, op_type: String, operands: OperandsList }
+}
+impl ASTNode {
+    fn debug_dump( &self ) {
+        self.debug_dump_impl( 1 );
+    }
+    fn debug_dump_impl( &self, depth: usize ) {
+        for _ in 0..depth {
+            print!("\t", );
+        }
+        match self {
+            &ASTNode::Leaf{ ref token_idx, ref value } => println!("Leaf( {} ) at #{} ( evaluate -> {:?} )", value, token_idx, self.evaluate()),
+            &ASTNode::Node{ ref token_idx, ref op_type, ref operands } => {
+                println!("Node( {} ) at #{} (evaluate -> {:?})", op_type, token_idx, self.evaluate());
+                for operand in operands {
+                    operand.debug_dump_impl( depth + 1 );
+                }
+            }
+        }
+    }
+
+    fn new_node( token_idx: usize, op_type: &String, operands: OperandsList ) -> Box<ASTNode> {
+        Box::new( ASTNode::Node{
+                token_idx: token_idx,
+                op_type: op_type.clone(),
+                operands: operands
+            } )
+    }
+    fn new_leaf( token_idx: usize, value: &String ) -> Box<ASTNode> {
+        Box::new( ASTNode::Leaf{ token_idx: token_idx, value: value.clone() } )
+    }
+    pub fn evaluate( &self ) -> Result<String, String> {
+        match self {
+            &ASTNode::Leaf{ ref value, .. } => Ok( value.clone() ),
+            &ASTNode::Node{ ref op_type, .. } =>
+                match self.operand_values() {
+                    Err( reason ) => Err( reason ),
+                    Ok( operand_values ) =>
+                        match op_type.as_ref() {
+                            "+" => infix_operator_two_ints( |a: i64, b: i64| Ok( a + b ), &operand_values ),
+                            "-" => infix_operator_two_ints( |a: i64, b: i64| Ok( a - b ), &operand_values ),
+                            "*" => infix_operator_two_ints( |a: i64, b: i64| Ok( a * b ), &operand_values ),
+                            "/" => infix_operator_two_ints(
+                                |a: i64, b: i64|
+                                    if b == 0 { Err("division by zero".to_string()) }
+                                    else { Ok( a / b ) },
+                                &operand_values ),
+                            "%" => infix_operator_two_ints(
+                                |a: i64, b: i64|
+                                    if b == 0 { Err("division by zero".to_string()) }
+                                    else { Ok( a % b ) },
+                                &operand_values ),
+
+                            "=" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a == b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a == b) ),
+                                    &operand_values
+                                ),
+                            "!=" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a != b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a != b) ),
+                                    &operand_values
+                                ),
+                            "<" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a < b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a < b) ),
+                                    &operand_values
+                                ),
+                            ">" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a > b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a > b) ),
+                                    &operand_values
+                                ),
+                            "<=" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a <= b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a <= b) ),
+                                    &operand_values
+                                ),
+                            ">=" => infix_operator_two_ints_or_two_strings(
+                                    |a: i64, b: i64| Ok( bool_as_int(a >= b) ),
+                                    |a: &String, b: &String| Ok( bool_as_string(a >= b) ),
+                                    &operand_values
+                                ),
+                            "length" => prefix_operator_length( &operand_values ),
+                            "index" => prefix_operator_index( &operand_values ),
+                            "substr" => prefix_operator_substr( &operand_values ),
+
+                            _ => Err(format!("operation not implemented: {}", op_type))
+                        }
+                }
+        }
+    }
+    pub fn operand_values( &self ) -> Result<Vec<String>, String> {
+        if let &ASTNode::Node{ ref operands, .. } = self {
+            let mut out = Vec::with_capacity( operands.len() );
+            for operand in operands {
+                match operand.evaluate() {
+                    Ok( value ) => out.push( value ),
+                    Err( reason ) => return Err( reason ),
+                }
+            }
+            Ok( out )
+        }
+        else { panic!("Invoked .operand_values(&self) not with ASTNode::Node") }
+    }
+}
+
+pub fn tokens_to_ast( maybe_tokens: Result< Vec<(usize, Token)>, String > ) -> Result<Box<ASTNode>, String> {
+    if maybe_tokens.is_err() { Err( maybe_tokens.err().unwrap() ) }
+    else {
+        let tokens = maybe_tokens.ok().unwrap();
+        let mut out_stack: TokenStack = Vec::new();
+        let mut op_stack: TokenStack = Vec::new();
+
+        for (token_idx, token) in tokens {
+            if let Err( reason ) = push_token_to_either_stack( token_idx, &token, &mut out_stack, &mut op_stack ) {
+                return Err( reason )
+            }
+        }
+        if let Err( reason ) = move_rest_of_ops_to_out( &mut out_stack, &mut op_stack ) {
+            return Err( reason )
+        }
+        assert!( op_stack.is_empty() );
+
+        maybe_dump_rpn( &out_stack );
+        let result = ast_from_rpn( &mut out_stack );
+        if !out_stack.is_empty() {
+            Err( "syntax error (fist RPN token does not represent expression AST's root)".to_string() )
+        }
+        else {
+            maybe_dump_ast( &result );
+            result
+        }
+    }
+}
+
+fn maybe_dump_ast( result: &Result< Box<ASTNode>, String > ) {
+    use std::env;
+    if let Ok( debug_var ) = env::var( "EXPR_DEBUG_AST" ) {
+        if debug_var == "1" {
+            println!("EXPR_DEBUG_AST");
+            match result {
+                &Ok( ref ast ) => ast.debug_dump(),
+                &Err( ref reason ) => println!("\terr: {:?}", reason),
+           }
+        }
+    }
+}
+
+fn maybe_dump_rpn( rpn: &TokenStack ) {
+    use std::env;
+    if let Ok( debug_var ) = env::var( "EXPR_DEBUG_RPN" ) {
+        if debug_var == "1" {
+            println!("EXPR_DEBUG_RPN");
+            for token in rpn {
+                println!("\t{:?}", token);
+            }
+        }
+    }
+}
+
+fn ast_from_rpn( rpn: &mut TokenStack ) -> Result<Box<ASTNode>, String> {
+    match rpn.pop() {
+        None => Err( "syntax error (premature end of expression)".to_string() ),
+
+        Some( (token_idx, Token::Value{ value }) ) =>
+            Ok( ASTNode::new_leaf( token_idx, &value ) ),
+
+        Some( (token_idx, Token::InfixOp{ value, .. }) ) =>
+            maybe_ast_node( token_idx, &value, 2, rpn ),
+
+        Some( (token_idx, Token::PrefixOp{ value, arity }) ) =>
+            maybe_ast_node( token_idx, &value, arity, rpn ),
+
+        Some( (token_idx, unexpected_token) ) =>
+            panic!("unexpected token at #{} {:?}", token_idx, unexpected_token),
+    }
+}
+fn maybe_ast_node( token_idx: usize, op_type: &String, arity: usize, rpn: &mut TokenStack ) -> Result< Box<ASTNode>, String > {
+    let mut operands = Vec::with_capacity( arity );
+    for _ in 0..arity {
+        match ast_from_rpn( rpn ) {
+            Err( reason ) => return Err( reason ),
+            Ok( operand ) => operands.push( operand ),
+        }
+    }
+    operands.reverse();
+    Ok( ASTNode::new_node( token_idx, op_type, operands ) )
+}
+
+fn move_rest_of_ops_to_out( out_stack: &mut TokenStack, op_stack: &mut TokenStack ) -> Result<(), String> {
+    loop {
+        match op_stack.pop() {
+            None => return Ok( () ),
+            Some( (token_idx, Token::ParOpen) ) => return Err( format!( "syntax error (Mismatched open-parenthesis at #{})", token_idx ) ),
+            Some( (token_idx, Token::ParClose) ) => return Err( format!( "syntax error (Mismatched close-parenthesis at #{})", token_idx ) ),
+            Some( other ) => out_stack.push( other )
+        }
+    }
+}
+
+fn push_token_to_either_stack( token_idx: usize, token: &Token, out_stack: &mut TokenStack, op_stack: &mut TokenStack ) -> Result<(), String> {
+    let result =
+        match token {
+            &Token::Value{ .. } => Ok( out_stack.push( (token_idx, token.clone()) ) ),
+
+            &Token::InfixOp{ .. } =>
+                if op_stack.is_empty() { Ok( op_stack.push( (token_idx, token.clone()) ) ) }
+                else { push_op_to_stack( token_idx, token, out_stack, op_stack ) },
+
+            &Token::PrefixOp{ .. } => Ok( op_stack.push( (token_idx, token.clone()) ) ),
+
+            &Token::ParOpen => Ok( op_stack.push( (token_idx, token.clone()) ) ),
+
+            &Token::ParClose => move_till_match_paren( out_stack, op_stack )
+        };
+    maybe_dump_shunting_yard_step( token_idx, token, out_stack, op_stack, &result );
+    result
+}
+
+fn maybe_dump_shunting_yard_step( token_idx: usize, token: &Token, out_stack: &TokenStack, op_stack: &TokenStack, result: &Result<(), String> ) {
+    use std::env;
+    if let Ok( debug_var ) = env::var( "EXPR_DEBUG_SYA_STEP" ) {
+        if debug_var == "1" {
+            println!("EXPR_DEBUG_SYA_STEP");
+            println!("\t{} => {:?}", token_idx, token);
+            println!("\t\tout: {:?}", out_stack);
+            println!("\t\top : {:?}", op_stack);
+            println!("\t\tresult: {:?}", result);
+        }
+    }
+}
+
+fn push_op_to_stack( token_idx: usize, token: &Token, out_stack: &mut TokenStack, op_stack: &mut TokenStack ) -> Result<(), String> {
+    if let &Token::InfixOp{ precedence: prec, left_assoc: la, .. } = token {
+        loop {
+            match op_stack.last() {
+                None =>
+                    return Ok( op_stack.push( (token_idx, token.clone()) ) ),
+
+                Some( &(_, Token::ParOpen) ) =>
+                    return Ok( op_stack.push( (token_idx, token.clone()) ) ),
+
+                Some( &(_, Token::InfixOp{ precedence: prev_prec, .. }) ) =>
+                    if la && prev_prec >= prec
+                    || !la && prev_prec > prec {
+                        out_stack.push( op_stack.pop().unwrap() )
+                    }
+                    else {
+                        return Ok( op_stack.push( (token_idx, token.clone()) ) )
+                    },
+
+                Some( &(_, Token::PrefixOp{ .. }) ) =>
+                    return Ok( op_stack.push( (token_idx, token.clone()) ) ),
+
+                Some( _ ) => panic!("Non-operator on op_stack")
+            }
+        }
+    }
+    else {
+        panic!("Expected infix-op")
+    }
+}
+
+fn move_till_match_paren( out_stack: &mut TokenStack, op_stack: &mut TokenStack ) -> Result<(), String> {
+    loop {
+        match op_stack.pop() {
+            None => return Err( "syntax error (Mismatched close-parenthesis)".to_string() ),
+            Some( (_, Token::ParOpen) ) => return Ok( () ),
+            Some( other ) => out_stack.push( other )
+        }
+    }
+}
+
+
+fn infix_operator_two_ints<F>( f: F, values: &Vec<String> ) -> Result<String, String>
+    where F : Fn( i64, i64 ) -> Result<i64, String>
+{
+    assert!( values.len() == 2 );
+    if let Some( left ) = values[0].parse::<i64>().ok() {
+        if let Some( right ) = values[1].parse::<i64>().ok() {
+            return match f( left, right ) {
+                Ok(result) => Ok(result.to_string()),
+                Err(reason) => Err(reason),
+            }
+        }
+    }
+    Err( "Expected an integer operand".to_string() )
+}
+
+
+fn infix_operator_two_ints_or_two_strings<FI, FS>( fi: FI, fs: FS, values: &Vec<String> ) -> Result<String, String>
+    where FI : Fn( i64, i64 ) -> Result<i64, String>,
+          FS : Fn( &String, &String ) -> Result<String, String>
+{
+    assert!( values.len() == 2 );
+    if let ( Some( a_int ), Some( b_int ) ) =
+        (
+            values[0].parse::<i64>().ok(),
+            values[1].parse::<i64>().ok()
+        ) {
+            match fi( a_int, b_int ) {
+                Ok( result ) => Ok(result.to_string()),
+                Err( reason ) => Err(reason)
+            }
+        }
+    else {
+        fs( &values[0], &values[1] )
+    }
+}
+
+fn prefix_operator_length( values: &Vec<String> ) -> Result<String, String> {
+    assert!( values.len() == 1 );
+    Ok( values[0].len().to_string() )
+}
+
+fn prefix_operator_index( values: &Vec<String> ) -> Result<String, String> {
+    assert!( values.len() == 2 );
+    let haystack = &values[0];
+    let needles = &values[1];
+
+    let mut current_idx = 0;
+    for ch_h in haystack.chars() {
+        current_idx += 1;
+
+        for ch_n in needles.chars() {
+            if ch_n == ch_h {
+                return Ok( current_idx.to_string() )
+            }
+        }
+    }
+    Ok( "0".to_string() )
+}
+
+fn prefix_operator_substr( values: &Vec<String> ) -> Result<String, String> {
+    assert!( values.len() == 3 );
+    let subj = &values[0];
+    let mut idx = match values[1].parse::<i64>() {
+        Ok( i ) => i,
+        Err( _ ) => return Err( "expected integer as POS arg to 'substr'".to_string() ),
+    };
+    let mut len = match values[2].parse::<i64>() {
+        Ok( i ) => i,
+        Err( _ ) => return Err( "expected integer as LENGTH arg to 'substr'".to_string() ),
+    };
+
+    if idx <= 0 || len <= 0 { return Ok( "".to_string() ) }
+
+    let mut out_str = String::new();
+    for ch in subj.chars() {
+        idx -= 1;
+        if idx <= 0 {
+            if len <= 0 { break; }
+            len -= 1;
+
+            out_str.push( ch );
+        }
+    }
+    Ok( out_str )
+}
+
+fn bool_as_int( b: bool ) -> i64 { if b { 1 } else { 0 } }
+fn bool_as_string( b: bool ) -> String { if b { "1".to_string() } else { "0".to_string() } }
+
diff --git a/src/expr/tokens.rs b/src/expr/tokens.rs
new file mode 100644
index 000000000..f1c0a65bf
--- /dev/null
+++ b/src/expr/tokens.rs
@@ -0,0 +1,160 @@
+/*
+ * This file is part of the uutils coreutils package.
+ *
+ * (c) Roman Gafiyatullin <r.gafiyatullin@me.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+//!
+//! The following tokens are present in the expr grammar:
+//! * integer literal;
+//! * string literal;
+//! * infix binary operators;
+//! * prefix operators.
+//!
+//! According to the man-page of expr we have expression split into tokens (each token -- separate CLI-argument).
+//! Hence all we need is to map the strings into the Token structures, except for some ugly fiddling with +-escaping.
+//!
+
+#[derive(Debug)]
+#[derive(Clone)]
+pub enum Token {
+    Value{ value: String },
+
+    ParOpen,
+    ParClose,
+
+    InfixOp {
+        precedence: u8,
+        left_assoc: bool,
+        value: String
+    },
+
+    PrefixOp {
+        arity: usize,
+        value: String
+    },
+}
+impl Token {
+    fn new_infix_op( v: &String, left_assoc: bool, precedence: u8 ) -> Self {
+        Token::InfixOp{
+            left_assoc: left_assoc,
+            precedence: precedence,
+            value: v.clone()
+        }
+    }
+    fn new_value( v: &String ) -> Self {
+        Token::Value{
+            value: v.clone()
+        }
+    }
+
+    fn is_infix_plus( &self ) -> bool {
+        match self {
+            &Token::InfixOp{ ref value, .. } => value == "+",
+            _ => false
+        }
+    }
+    fn is_a_number( &self ) -> bool {
+        match self {
+            &Token::Value{ ref value, .. } =>
+                match value.parse::<i64>() {
+                    Ok( _ ) => true,
+                    Err( _ ) => false
+                },
+            _ => false,
+        }
+    }
+    fn is_a_close_paren( &self ) -> bool {
+        match self {
+            &Token::ParClose => true,
+            _ => false,
+        }
+    }
+}
+
+pub fn strings_to_tokens( strings: &Vec<String> ) -> Result< Vec<(usize, Token)>, String > {
+    let mut tokens_acc = Vec::with_capacity( strings.len() );
+    let mut tok_idx = 1;
+
+    for s in strings {
+        let token_if_not_escaped =
+            match s.as_ref() {
+                "(" => Token::ParOpen,
+                ")" => Token::ParClose,
+
+                "^" => Token::new_infix_op( &s, false, 7 ),
+
+                ":" => Token::new_infix_op( &s, true, 6 ),
+
+                "*" => Token::new_infix_op( &s, true, 5 ),
+                "/" => Token::new_infix_op( &s, true, 5 ),
+                "%" => Token::new_infix_op( &s, true, 5 ),
+
+                "+" => Token::new_infix_op( &s, true, 4 ),
+                "-" => Token::new_infix_op( &s, true, 4 ),
+
+                "=" => Token::new_infix_op( &s, true, 3 ),
+                "!=" => Token::new_infix_op( &s, true, 3 ),
+                "<" => Token::new_infix_op( &s, true, 3 ),
+                ">" => Token::new_infix_op( &s, true, 3 ),
+                "<=" => Token::new_infix_op( &s, true, 3 ),
+                ">=" => Token::new_infix_op( &s, true, 3 ),
+
+                "&" => Token::new_infix_op( &s, true, 2 ),
+
+                "|" => Token::new_infix_op( &s, true, 1 ),
+
+                "match" => Token::PrefixOp{ arity: 2, value: s.clone() },
+                "substr" => Token::PrefixOp{ arity: 3, value: s.clone() },
+                "index" => Token::PrefixOp{ arity: 2, value: s.clone() },
+                "length" => Token::PrefixOp{ arity: 1, value: s.clone() },
+
+                _ => Token::new_value( &s ),
+            };
+        push_token_if_not_escaped( &mut tokens_acc, tok_idx, token_if_not_escaped, &s );
+        tok_idx += 1;
+    }
+    maybe_dump_tokens_acc( &tokens_acc );
+
+    Ok( tokens_acc )
+}
+
+fn maybe_dump_tokens_acc( tokens_acc: &Vec<(usize, Token)> ) {
+    use std::env;
+
+    if let Ok(debug_var) = env::var( "EXPR_DEBUG_TOKENS" ) {
+        if debug_var == "1" {
+            println!("EXPR_DEBUG_TOKENS");
+            for token in tokens_acc {
+                println!("\t{:?}", token);
+            }
+        }
+    }
+}
+
+fn push_token_if_not_escaped( acc: &mut Vec<(usize, Token)>, tok_idx: usize, token: Token, s: &String ) {
+    // Smells heuristics... :(
+    let prev_is_plus =
+        match acc.last() {
+            None => false,
+            Some( ref t )  => t.1.is_infix_plus(),
+        };
+    let should_use_as_escaped =
+        if prev_is_plus && acc.len() >= 2 {
+            let pre_prev = &acc[acc.len() - 2];
+            ! ( pre_prev.1.is_a_number() || pre_prev.1.is_a_close_paren() )
+        }
+        else if prev_is_plus { true }
+        else { false };
+
+    if should_use_as_escaped {
+        acc.pop();
+        acc.push( (tok_idx, Token::new_value( s )) )
+    }
+    else {
+        acc.push( (tok_idx, token) )
+    }
+}