/**
 * @name Replacement of a substring with itself
 * @description Replacing a substring with itself has no effect and may indicate a mistake.
 * @kind problem
 * @problem.severity warning
 * @security-severity 7.8
 * @id js/identity-replacement
 * @precision very-high
 * @tags correctness
 * security
 * external/cwe/cwe-116
 */

import javascript

/**
 * Holds if `e`, when used as the first argument of `String.prototype.replace`, matches
 * `s` and nothing else.
 */
predicate matchesString(Expr e, string s) {
 exists(RegExpLiteral rl |
 rl.flow().(DataFlow::SourceNode).flowsToExpr(e) and
 not rl.isIgnoreCase() and
 regExpMatchesString(rl.getRoot(), s)
 )
 or
 s = e.getStringValue()
}

/**
 * Holds if `t` matches `s` and nothing else.
 */
language[monotonicAggregates]
predicate regExpMatchesString(RegExpTerm t, string s) {
 t.isPartOfRegExpLiteral() and
 (
 // constants match themselves
 s = t.(RegExpConstant).getValue()
 or
 // assertions match the empty string
 (
 t instanceof RegExpCaret or
 t instanceof RegExpDollar or
 t instanceof RegExpWordBoundary or
 t instanceof RegExpNonWordBoundary or
 t instanceof RegExpLookahead or
 t instanceof RegExpLookbehind
 ) and
 s = ""
 or
 // groups match their content
 regExpMatchesString(t.(RegExpGroup).getAChild(), s)
 or
 // single-character classes match that character
 exists(RegExpCharacterClass recc | recc = t and not recc.isInverted() |
 recc.getNumChild() = 1 and
 regExpMatchesString(recc.getChild(0), s)
 )
 or
 // sequences match the concatenation of their elements
 exists(RegExpSequence seq | seq = t |
 s =
 concat(int i, RegExpTerm child |
 child = seq.getChild(i)
 |
 any(string subs | regExpMatchesString(child, subs)) order by i
 )
 )
 )
}

from MethodCallExpr repl, string s, string friendly
where
 repl.getMethodName() = ["replace", "replaceAll"] and
 matchesString(repl.getArgument(0), s) and
 repl.getArgument(1).getStringValue() = s and
 (if s = "" then friendly = "the empty string" else friendly = "'" + s + "'")
select repl.getArgument(0), "This replaces " + friendly + " with itself."