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
  • Non-capturing group
    (?:...)
  • Capturing group
    (...)
  • 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 = /^[sS]ee, e\.g\.,(?![^()\n]*(?=\()(\(1st Cir\. \d{4}\))?[^()\n]*(?=\(|.)(\(2nd Cir\. \d{4}\))?[^()\n]*(?=\(|.)(\(3rd Cir\. \d{4}\))?[^()\n]*(?=\(|.)(\(4th Cir\. \d{4}\))?[^()\n]*(?=\(|.)(\(5th Cir\. \d{4}\))?[^()\n]*(?=\(|.)(\(D\.C\. Cir\. \d{4}\))?\.?$).*/gm; // Alternative syntax using RegExp constructor // const regex = new RegExp('^[sS]ee, e\\.g\\.,(?![^()\\n]*(?=\\()(\\(1st Cir\\. \\d{4}\\))?[^()\\n]*(?=\\(|.)(\\(2nd Cir\\. \\d{4}\\))?[^()\\n]*(?=\\(|.)(\\(3rd Cir\\. \\d{4}\\))?[^()\\n]*(?=\\(|.)(\\(4th Cir\\. \\d{4}\\))?[^()\\n]*(?=\\(|.)(\\(5th Cir\\. \\d{4}\\))?[^()\\n]*(?=\\(|.)(\\(D\\.C\\. Cir\\. \\d{4}\\))?\\.?$).*', 'gm') const str = `See, e.g., Smith v. U.S. (1st Cir. 2012); U.S. v. Sara (3rd Cir. 2017). See, e.g., Jefferson v. U.S. (2nd Cir. 2012); U.S. v. Coolidge (D.C. Cir. 2017). See, e.g., Lincoln v. Jones (3rd Cir. 2012); U.S. v. Roosevelt (5th Cir. 2017). See, e.g., Smith v. U.S. (5th Cir. 2012); U.S. v. Sara (1st Cir. 2017). See, e.g., Jefferson v. U.S. (D.C. Cir. 2012); U.S. v. Coolidge (10th Cir. 2017). See, e.g., Lincoln v. Jones (9th Cir. 2012); U.S. v. Roosevelt (3d Cir. 2017).`; // Reset `lastIndex` if this regex is defined globally // regex.lastIndex = 0; 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