What Is a Regular Expression
A regular expression (abbreviated as regex) is a mini-language for describing string patterns. A few lines of regex can accomplish text-processing tasks like "find all email addresses," "validate phone number format," or "replace all date formats."
Basic Syntax
Character Classes
Use square brackets to match any one of a set of characters:
[abc]matches a or b or c[a-z]matches any lowercase letter[0-9]matches any digit, equivalent to\d[^abc]matches any character except a/b/c
Predefined Characters
\ddigit,\Dnon-digit\wletter, digit, or underscore,\Wopposite\swhitespace,\Snon-whitespace.any character (except newline)
Quantifiers
*zero or more times+one or more times?zero or one time{n}exactly n times,{n,m}n to m times
Grouping and Anchors
(...)capturing group^start of line,$end of line
Common Examples
Email Validation
^[\w.-]+@[\w.-]+\.\w{2,}$
Chinese Mobile Number
^1[3-9]\d{9}$
URL Extraction
https?://[\w./?=&%-]+
DocsAll Regex Testing Tool
The most effective way to learn regex is to write and test simultaneously. DocsAll provides an online regex testing tool, no environment setup required:
1. Enter the Regex
Type the regex in the expression input box and select modifiers (g global, i case-insensitive, m multiline).
2. Enter Test Text
Paste the content to match in the text box below.
3. View Results in Real Time
Matched content is highlighted, and all capture groups are listed.
Learning Tips
- Start with simple examples, don't write complex regex right away
- Use groups and named groups to improve readability
- Remember the difference between greedy and non-greedy (
*?+?) - Avoid nested quantifiers in performance-sensitive scenarios
Regular expressions seem hard to learn, but after mastering the basic syntax, practice more with the DocsAll regex testing tool and you'll become proficient within a week.