Added basic regex parser

Author: rampatra

src/main/java/com/rampatra/strings/BasicRegexParser.java

@@ -0,0 +1,59 @@
+package com.rampatra.strings;
+
+/**
+ * @author rampatra
+ * @since 25/11/2018
+ */
+public class BasicRegexParser {
+
+    /**
+     * Implement a regular expression function isMatch that supports the '.' and '*' symbols.
+     * The function receives two strings - text and pattern - and should return true if the
+     * text matches the pattern as a regular expression. For simplicity, assume that the actual
+     * symbols '.' and '*' do not appear in the text string and are used as special symbols only
+     * in the pattern string.
+     *
+     * @param text
+     * @param pattern
+     * @return
+     */
+    private static boolean isMatch(String text, String pattern) {
+        int textIndex = 0;
+        int patternIndex = 0;
+
+        while (textIndex < text.length()) {
+            if (patternIndex >= pattern.length()) return false;
+
+            if (pattern.charAt(patternIndex) == '.' || text.charAt(textIndex) == pattern.charAt(patternIndex)) {
+                // if the next character in the pattern is a '*' then forward the text pointer
+                if (patternIndex < pattern.length() - 1 && pattern.charAt(patternIndex + 1) == '*') {
+                    while (textIndex < text.length() - 1 && text.charAt(textIndex + 1) == text.charAt(textIndex)) {
+                        textIndex++;
+                    }
+                    patternIndex++; // for the '*' in the pattern
+                }
+            } else if (patternIndex < pattern.length() - 1 && pattern.charAt(patternIndex + 1) == '*') { // if the text
+                // and pattern are not equal at the index but the pattern has a '*' after then increment the pattern pointer
+                textIndex--; // as we incrementing the text pointer outside but we should, in fact, stay at the current text index
+                patternIndex++;
+            } else {
+                return false;
+            }
+            textIndex++;
+            patternIndex++;
+        }
+        return true;
+    }
+
+    public static void main(String[] args) {
+        System.out.println(isMatch("aa", "a")); // output: false
+        System.out.println(isMatch("aa", "aa")); // output: true
+        System.out.println(isMatch("abc", "a.c")); // output: true
+        System.out.println(isMatch("abbb", "ab*")); // output: true
+        System.out.println(isMatch("acd", "ab*c.")); // output: true
+        System.out.println(isMatch("abbdbb", "ab*d")); // output: false
+        System.out.println(isMatch("aba", "a.a")); // output: true
+        System.out.println(isMatch("acd", "ab*c.")); // output: true
+        System.out.println(isMatch("abaa", "a.*a*")); // output: true
+    }
+}