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+#!/usr/bin/env python3
+
+# Challenge 287
+#
+# Task 2: Valid Number
+# Submitted by: Mohammad Sajid Anwar
+#
+# You are given a string, $str.
+#
+# Write a script to find if it is a valid number.
+#
+# Conditions for a valid number:
+#
+# - An integer number followed by an optional exponent.
+# - A decimal number followed by an optional exponent.
+# - An integer number is defined with an optional sign '-' or '+' followed by digits.
+#
+# Decimal Number:
+#
+# A decimal number is defined with an optional sign '-' or '+' followed by one
+# of the following definitions:
+# - Digits followed by a dot '.'.
+# - Digits followed by a dot '.' followed by digits.
+# - A dot '.' followed by digits.
+#
+# Exponent:
+#
+# An exponent is defined with an exponent notation 'e' or 'E' followed by an integer number.
+#
+# Example 1
+#
+# Input: $str = "1"
+# Output: true
+#
+# Example 2
+#
+# Input: $str = "a"
+# Output: false
+#
+# Example 3
+#
+# Input: $str = "."
+# Output: false
+#
+# Example 4
+#
+# Input: $str = "1.2e4.2"
+# Output: false
+#
+# Example 5
+#
+# Input: $str = "-1."
+# Output: true
+#
+# Example 6
+#
+# Input: $str = "+1E-8"
+# Output: true
+#
+# Example 7
+#
+# Input: $str = ".44"
+# Output: true
+
+import re
+import sys
+
+if re.search(r'  ^ [-+]?                '+
+             r'    (?: \d+ \. \d* |     '+
+             r'        \d* \. \d+ |     '+
+             r'        \d+              '+
+             r'    )                    '+
+             r'    (?: e [-+]? \d+ )?   '+
+             r'  $', sys.argv[1], re.X+re.I):
+    print('true')
+else:
+    print('false')