Regular Expressions 101

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An explanation of your regex will be automatically generated as you type.
Detailed match information will be displayed here automatically.
  • All Tokens
  • Common Tokens
  • General Tokens
  • Anchors
  • Meta Sequences
  • Quantifiers
  • Group Constructs
  • Character Classes
  • Flags/Modifiers
  • Substitution
  • A single character of: a, b or c
    [abc]
  • A character except: a, b or c
    [^abc]
  • A character in the range: a-z
    [a-z]
  • A character not in the range: a-z
    [^a-z]
  • A character in the range: a-z or A-Z
    [a-zA-Z]
  • Any single character
    .
  • Alternate - match either a or b
    a|b
  • Any whitespace character
    \s
  • Any non-whitespace character
    \S
  • Any digit
    \d
  • Any non-digit
    \D
  • Any word character
    \w
  • Any non-word character
    \W
  • Match everything enclosed
    (?:...)
  • Capture everything enclosed
    (...)
  • Zero or one of a
    a?
  • Zero or more of a
    a*
  • One or more of a
    a+
  • Exactly 3 of a
    a{3}
  • 3 or more of a
    a{3,}
  • Between 3 and 6 of a
    a{3,6}
  • Start of string
    ^
  • End of string
    $
  • A word boundary
    \b
  • Non-word boundary
    \B

Regular Expression
No Match

/
/
gm

Test String

Code Generator

Language

Generated Code

const regex = /^[a-zA-Z]\d{4}$/gm; // Alternative syntax using RegExp constructor // const regex = new RegExp('^[a-zA-Z]\\d{4}$', 'gm') const str = `# The CMS, who maintain these codes, describe the format as:- # "HCPCS Level II codes (also known as alpha-numeric codes) consist of a single alphabetical letter followed by 4 numeric digits." # (https://www.cms.gov/medicare/coding-billing/healthcare-common-procedure-system) # NOTE: This regex will not identify HCPCS Level I codes (AKA CPT codes) # Also, some sources say the single letter ranges only from A-V (this regex follows A-Z) # Finally, this regex does not match procedure modifiers, which are 2 digit codes that follow a HCPCS code (with hyphen). # (https://www.aapc.com/resources/what-are-medical-coding-modifiers) # A separate regex has been created on regex101 supporting these modifiers # Genuine codes E8015 A6410 C5278 G2102 M1221 V2623 V2756 c5278 g2102 m1221 v2623 v2756 # Should fail EE8015 6410 E641O [6410 _6510 g212 C52789 `; let m; while ((m = regex.exec(str)) !== null) { // This is necessary to avoid infinite loops with zero-width matches if (m.index === regex.lastIndex) { regex.lastIndex++; } // The result can be accessed through the `m`-variable. m.forEach((match, groupIndex) => { console.log(`Found match, group ${groupIndex}: ${match}`); }); }

Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions