The basic rules of regular expressions

Many of you are familiar with wild cards (in the Unix world, it is called glob pattern) matching of text. Here:

• ? matches any single character
• * matches any sequence of characters
• [abc] matches any one character inside square brackets, so it will match a, b, or c

The regular expression pattern goes many steps farther than wild cards, where one can set many rules in a regex pattern, such as the following:

• Match a character or a group of characters optionally (0 or 1 times)
• Use quantifiers in regex patterns to match variable length text
• Use a character class to match one of the listed characters or match a range of characters
• Use a negated character class to match any character except those matched by the character class
• Match only certain character categories, such as match only digits, only upper case letters, or only punctuation characters
• Match a character or a group of characters for a specific length.
• Match a length range, such as allow only six to 10 digits in the input or match an input of a minimum of eight characters
• Use Boolean "OR" in an alternation to match one of the few alternative options
• Use groups in regex patterns and capture substrings that we want to extract or replace from a given input
• Alter the behavior of matching by keeping it greedy (eager), lazy (reluctant), or possessive
• Use back references and forward references of groups that we capture
• Use zero-width assertions such as the following:
  • Start and end anchors
  • Word boundary
  • Lookahead and lookbehind assertions
  • Start a match from the end of a previous match

For example, in a regex to match a or b we can use the following alternation:

a|b

To match one or more instances of the digit 5, we can use the following:

5+

To match any substring that starts with p and ends with w, we can use the following:

p.*w