look behinds

- Closes #2

Author: mfix22

README.md

@@ -75,13 +75,15 @@ Similar to a capture(...), but won't keep the capture within the parentheses
 group('.|\\s') // -> (?:.|\\s)
 ```
 
-### `look.ahead.(positive|negative)`
+### `look.(ahead|behind).(positive|negative)`
 
 Creates a [negative or positive look-ahead](https://www.stefanjudis.com/today-i-learned/the-complicated-syntax-of-lookaheads-in-javascript-regular-expressions/)
 
 ```javascript
-look.ahead.positive('Y') // -> '(?=y)'
+look.ahead.positive('Y') === look.ahead('Y') // -> '(?=y)'
 look.ahead.negative('Y') // -> '(?!y)'
+look.behind.positive('Y') === look.behind('Y') // -> '(?<=y)'
+look.behind.negative('Y') // -> '(?<!y)'
 ```
 
 #### `regex`

index.js

@@ -37,7 +37,8 @@ const group = v => (v && v.length ? `(${GROUP}${v})` : '')
 
 const ALL = capture(or(ANY, WHITE_SPACE)) // matches any character or whitespace
 
-const look = posOrNeg => text => `(?${posOrNeg ? '=' : '!'}${text})`
+const look = (posOrNeg, behindOrAhead) => text =>
+  `(?${behindOrAhead ? '<' : ''}${posOrNeg ? '=' : '!'}${text})`
 
 const regex = (...args) => new RegExp(...args)
 
@@ -53,9 +54,13 @@ module.exports = exports.default = {
   capture,
   group,
   look: {
-    ahead: Object.assign(look(true), {
-      positive: look(true),
-      negative: look(false)
+    ahead: Object.assign(look(true, false), {
+      positive: look(true, false),
+      negative: look(false, false)
+    }),
+    behind: Object.assign(look(true, true), {
+      positive: look(true, true),
+      negative: look(false, true)
     })
   },
   matchers: {

index.spec.js

@@ -53,8 +53,12 @@ describe('rexrex', () => {
     expect(repeat('8', 1, 2)).toEqual('8{1,2}')
     expect(repeat('8', 1, Infinity)).toEqual('8{1,}')
   })
-  test('Look ahead', () => {
+  test('Look ahead/behind', () => {
+    expect(look.ahead('8')).toEqual('(?=8)')
     expect(look.ahead.positive('8')).toEqual('(?=8)')
     expect(look.ahead.negative('8')).toEqual('(?!8)')
+    expect(look.behind('8')).toEqual('(?<=8)')
+    expect(look.behind.positive('8')).toEqual('(?<=8)')
+    expect(look.behind.negative('8')).toEqual('(?<!8)')
   })
 })