Master Common Regex Patterns: Digits, Times, IPs, and HTML Tags

Simple Regex Exercises

A set of six practice tasks covering: (1) matching the first three digits at the start of a string, (2) matching any character except a, b or c, (3) the result of '1234567'.match(/\d{1,3}/g), (4) matching words that do not begin with th, (5) a password rule of 4‑8 characters with at least one digit, and (6) matching a single Chinese character.

Answers

Match three digits at the start: /^\d{3}/ Match any character except a, b, c: /[^abc]/ Result of the greedy match: ["123", "456", "7"] Match words not beginning with th: /\b(?!th)\w+\b/ Password rule (4‑8 chars, at least one digit): /^(?=.*\d).{4,8}$/ Match a Chinese character:

/[\u4e00-\u9fa5]/

Principles of Good Regexes

Match only the desired text and exclude unwanted text.

Keep the expression easy to understand and maintain.

Ensure efficiency.

Matching Dollar Amounts

The pattern /^\$[0-9]+(\.[0-9][0-9])?$/ matches a leading dollar sign, one or more digits, and an optional decimal part with exactly two digits. It does not match formatted amounts such as $1,000.

Matching 24‑Hour Time (e.g., 09:59)

Two equivalent approaches: /\b(?:[01]?[0-9]|2[0-3]):[0-5][0-9]\b/ or /\b(?:[012]?[0-3]|[01]?[4-9]):[0-5][0-9]\b/ For full‑string validation replace the leading \b with ^ and the trailing \b with $. When used for search‑replace be aware that the pattern may also match substrings like "19:50" inside longer text.

Matching IPv4 Addresses

A field representing a number from 0 to 255 can be expressed as: [01]?\d\d?|2[0-4]\d|25[0-5] Combining four such fields yields the full address regex:

/^(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])$/

To reject the special case 0.0.0.0 add a negative look‑ahead:

/^(?!0+\.0+\.0+\.0+$)(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])\.(?:[01]?\d\d?|2[0-4]\d|25[0-5])$/

Matching Text Between Delimiters

Quoted Strings Allowing Escaped Quotes

A robust pattern that handles escaped quotes inside double‑quoted strings:

/"(?:\\.|[^\\"])*"/

HTML Tag Matching

Pattern that matches an HTML tag where attribute values may be double‑quoted, single‑quoted, or unquoted, and allows the > character inside attribute values:

/<(?:"[^"]*"|'[^']*'|[^'">])*>/

Reference

Mastering Regular Expressions – http://imweb.io/topic/56e804ef1a5f05dc50643106